tldr: boxes from cake shops, then different ways (bags, straps, string) & considerations for carrying them (if all going to one place or multiple, if you're carrying other stuff, too)
Cake shops, if they sell supplies, will sell boxes of various sizes. Get 6 of them just large enough to fit your pies (meringue or other tall pies will need a taller box). Once the pies are in boxes things become much, much easier. You might have a gym bag that's large enough to fit them all laying flat. You might have some large grocery bags where they can safely fit flat for you.
If you can't find anything suitable, you can stack them up and tie them together. Make sure to leave enough string to tie loops at the top as carrying handles (and make sure they're well-tied). Things fare much better when held like a pendulum -- if they get bumped, they swing rather than smashing up against the box. I've also been known to put a square of anti-slip shelf liner under cake boards when transporting them, to lower the risk
Load the pies into the boxes, stack them up, and tie them together. Once it's tied well, use the excess string to tie loops for a handle at the top, and make sure it's tied well on both ends.
Depending on the weight of the pies, and what else you'll be carrying, you might want to tie them into one large stack, but when carrying things, I like two even stacks, so you're well balanced. (if you're carrying other stuff, the other stuff might balance against the one big stack).
The smaller stacks can also be useful if you're going to be dropping off things at two different locations.
If you're dropping each one off at a different location, then are much easier than tying. If you have to use string, you may want to bundle up a single box with carrying loops, then tie in the second box below it, and then a third box. You might also be able to use a yoga strap or something similar to attach the lower boxes to the top that you can re-adjust without too much difficulty. (It might also be able to tie a loop in it, but it depends on the length of the straps that you have available).
If you're going to be doing this a lot, I'd recommend looking at grocery stores that have the re-usable bags and if they have large enough ones, get one or two. (IKEA has huge bags for under $1. BJs also has some larger bags (not as large as the IKEA ones. I suspect other warehouse type stores would have some, too).
If you're going to be carrying larger things, and don't have an IKEA nearby, you might want to consider something like the Velcro All-Purpose Strap with Handle, which lets you cinch the strap around something to be carried, and then has a built in handle. (they used to have padded handles, but it doesn't look like it from that picture ... I guess it might've been a different brand). They do have a minimum size that they can go down to, though. (a little more than 1/2 the maximum size. At 1/2 maximum size, only 1/2 of the velcro is making contact).