All the online sites have them, like the knot shop, weddingchannel.com, etc. You can also find them in craft stores and party stores. But they can run pretty expensive, and I had trouble finding a pretty simple looking one in ivory. My florist is ending up just supplying my basket with the arrangement (she is even adding some of the ribbon I''m using as accents).
I don''t think it would be too hard to make if you could find a plain basket in the right size. Then just give it some "pretties" with ribbons or rosettes or something.