Where do I purchase a fly with the collagen tagged?
There are some more places other than what Bez has already mentioned.

See here.

However, tagging collagen may be a problem. Collagen undergoes a lot of post-translational modifications including hydroxylation and end clipping. It forms a unique structure (triplex) and adding tags may compromise its structural integrity. You can try Masson's trichome stain instead, which is used to stain collagen — a very cheap alternative.