Can I get collagen injections for buttocks?

Collagen injections are rarely if ever done any more. For localized indents, hyaluronic acids such as Juverderm or Restylane might give you a good result, or even fat injections if the indents are not too small. These would not be permanent solutions but temporary fixes. Sometimes the filler moves out to the sides and lifts the normal skin but the indent doesn't change. It can look more obvious if the surrounding skin lifts from the spread. Try one as a test and if it looks good after a few weeks (retaining its good effect) then consider doing the others.

If your question is strictly about the use of Collagen (Cow / Human) to correct cellulite dimples -- I would say probably no. A 1cc syringe of Collagen costs a few hundred dollars and, depending on the amount of filler needed, this can become very expensive. In addition, collagen is not the best filler for this use.
Other fillers, especially your own fat, may be used. The extreme example of this is the Brazilian Butt Lift in which the buttocks are augmented and contoured with fat transfers.

Cellulite has no known correction though many things have been tried. At their best, those treatments that seemed promising maybe just caused swelling in the tissues that disguised the dimpling.
Collagen is a short lived, allergy prone filler than has slipped onto the back burner of injectable filler options. Even so, no filler could fix the problem you describe even if you were willing to spend thousands each year having it done and re-done as the material dissolves.