Where can you buy E. coli bacteria?

You can buy bacteria (and also yeasts, molds, and viruses) from institutions called type culture collections and from some commercial sources that supply high school and university Biology classes. Here are some sources for the USA:

ATCC (American Type Culture Collection) http://www.atcc.org This is the authoritative source, but it serves research institutions predominantly and is pretty expensive. Supply of anything to individuals is restricted.

Presque Isle Cultures http://www.picultures.com has a small collection and sells non-pathogenic strains also to individuals.

Carolina Biological Supply also sells a few bacteria including E. coli:

http://www.carolina.com/product/escherichia+coli+b%2C+living.do?keyWord=coli&sortby=bestMatches