What do I look for in a good high quality apron?
Where can I go to get one?
What sort of tasks are you looking to protect yourself from?
If it's really messy stuff, or to wear while doing the dishes, you might want something that's non-absorbant, like PVC. I've only really seen them at restaurant supply stores, but it's possible that you might be able to get them from chemistry supply places, also. (and in this case, pockets are not a good idea, as they collect stuff that you've then got to clean out).
For long hours of grilling, I like something that's more insulating, which in my case would be a leather welder's apron, which you can get from welding supply stores. I've looked around for something that might be more reflective of radiant heat, but haven't had any luck.
For just general practical use, something that cleans up easily is important. Restaurant supply stores are a good bet for those sorts of things, but you can sometimes find them in stores that sell grills when it's that season.
The other more frilly decorative ones ... you can find at most housewares stores, some department stores, and if you want something more one of a kind, try esty.
... as for features ... I'm not a fan of pockets, myself. I guess they're useful for thermometers and timers, but mine clip on, or sit up on a counter or next to the grill, so I don't care. What I do care about is that they cover the areas that you're trying to protect (an adjustable neck strap is so much better than tying a knot in the line around the neck so try to shorten it) and they don't have straps so long that they get snagged if I sit down for lean up against something.
And it should come on and off without too much difficulty ... which I've only really noticed is a problem with some designs of welder's aprons (it's hard to explain ... it doesn't just go over the neck, and attach in back) ... or if you have to tie off the top to shorten the neck opening.
It depends on what you want, really. If you want something pretty and frilly, those are pretty easy to find anywhere. If you want something a bit stronger, easy to clean, and that will stand up to more than the average "for-looks" apron, I would suggest looking at a kitchen-supply store, the sort that sells various kitchen supplies and gadgets. You should be able to find a heavier apron with pockets - I have a heavier cotton one with adjustable straps and a couple of pockets that I found in the kitchen section at Zellers, for example.
The primary purpose of an apron is to keep you clean, because cooking gets you dirty. This is why I don't like the common cotton aprons you can get everywhere, they absorb dirt and keep stains. If you insist on cotton, get a white or natural colored one (not dyed, and certainly not printed) so you can wash it at 90° and use bleach and other stain removing chemicals.
What I prefer are treated aprons. They are made from the same material as some hiking jackets, and repel dirt and water. You cannot wash them at too high a setting, but you don't need that. Just remember to not use fabric softener on them, and renew the coating every month or two using an impregnating spray for outdoor clothing. The one I like is by Le Creuset, with an adjustable neck strap and a clean design which covers a lot of body area, and you can easily get it from Amazon. It also has pockets, which I find essential for stuffing a pair of pot holders - when I suddenly smell something burning, I have no time for searching for them somewhere else. But I imagine there are other brands which will do as well. If you insist on having a witty saying on your apron, it is probably better to get a high-quality coated apron and have it screen printed or stitched than buying a poorly functioning complete with a saying.
This is the Le Creuset apron:
I didn't know that you can get pure PVC aprons until I read Joe's answer. I can imagine they will be sweat-inducing, because they are not breathable, but as I know PVC, they will also be even easier to clean than coated cloth. He says they are hard to get, but I can tell you that aprons are one of the easiest things to sew, so if you want one badly, why not just buy some PVC table cloth and make one? (I am assuming here that you can get your hands on a sewing machine; you don't need much experience in sewing for making an apron, and cheap/free patterns for aprons are easy to come by).
For special needs, there are all kinds of aprons, but I don't think you need them as a hobby cook. You can get protection from extreme heat, acids/bases, shards flying through the air (as in hacking wood to kindling, or in smallish explosions), or even radiation, but I hope you have none of those in your kitchen. If you just want a high show off factor, take a look here. That's a butcher's apron from aluminum plates by De Buyer. If you get it complete with the chain mail glove (seriously), you are ready for the next ren faire. .
I have a cotton one as well, has a large pocket across the front, over the head and around the waist straps. I'd just look for a good sturdy fabric that looks well sewn together. Heavy use will get it stained with all sorts of things so any apron has to have the ability to withstand many trips through the washing machine.
My primary apron is black in color, black tends to hide most of the stains (except when I'm making bread).
And completely off topic ... my apron says "The SPICE must flow" .... there ya go ... Dune reference and cooking reference all in one!
gettings one is easy, check out pretty much any store that sells cooking stuff
I like mine to be made of cotton-it's easy to clean and is soft so it doesn't irritate your body. I like mine to have a strap for my neck and a tie around my waist. I also like having pockets to stash stuff like spoons.