I'm in need of a food-safe gasketing material for a maple sap-boiling setup. It needs to be squishy like vinyl tubing but I want to make sure it won't melt or decompose or react with steam.
Most food-grade gaskets for boiling+ temps and steam applications are Silicone or EPDM, because they are flexible, inexpensive, take very high temperatures, and won't leak weird flavors. EPDM is considered the best for true steam applications. Buna-N (nitrile) and Viton would work as well, but may not hold up as long under steam pressure. For intermittent use, any of the four materials would work.
There are official "food grade" gaskets, but unless you are making product for commercial sale then you don't need products that have been FDA approved - just wash the gasket before installing to remove the packing lubricant.
I wouldn't bother with any big-box hardware stores, though some "specialty" hardware stores or home-brew shops may have what you need. Online is probably your best bet, however.
You might find these guys handy, retailers of many gaskets needed for the brewing industry, many of which may be applicable to you (particularly the standard DIN-style gaskets): http://www.brewerygaskets.com/
Also useful, EMI Supply sells these little tubes of FDA approved silicon sealant (more like a caulk): http://www.emisupply.com/catalog/product_info.php?products_id=9742
And for general purpose gaskets of every type imaginable, McMasterCarr: http://www.mcmaster.com/#gaskets/=gc516x
If you have more details on the dimensions needed, or the types of surfaces you are mating (including a picture?) it would be easier to choose a direction to go.
Silicone. They surely go up to 250°C (but check the package first; mine goes up to 250°C). Or is that too squishy?
You could also just ask a DIY store.
Does it have to be a squishy material? If you just need it to have a specific shape, there are some proven ways of doing this with hard/hardening materials that are high-temp safe.
Glass was the first to come to mind, but that's not exactly easy to mold. Clay, on the other hand, can be made to a certain shape and baked in an oven.
You can also go with molded metal pipes, soldered together with silver-based solder (as opposed to mercury).
If you want something less custom and are willing to pay, there are water distiller kits out there, though I don't know if they'd hit 105 C.