flexible food grade plastics that won't melt at 105 C
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.