Where to buy food chemicals
A quick Google search turns up a couple of companies. I didn't look for locations since you didn't specify yours or whether you're open to international shipping options.

This one looks like it deals in rather large quantities, some as low as one kilogram, others as much as a ton. Seems like a larger investment than you might be prepared for; you'd probably want to be reasonably sure of what you're doing. On the other hand, plenty of room for experimentation if it's in your budget.
http://www.alibaba.com/showroom/food-grade-chemicals.html

This one is obviously UK by its domain extension; it has a variety of food-grade chemicals
http://www.reagent.co.uk/sodium-metabisulphite-food-grade

Disclaimer: I know nothing at all about the quality, reputation, etc. of any of these; never met anyone who works in the field. My only connection and contribution here is a bit of Google-fu.

You might also check with your local pharmacies and health food stores; even if they don't sell in large enough quantities for a stop-and-shop situation they may be willing to special order for you, and possibly even negotiate a better price than retail. 

And since this isn't exactly a regular part of a pharmacy's business model/revenue source, they may even point you toward their suppliers for a direct connection.