letters of rec

No. You can only receive letters of rec from Alum!

It's going to depend on the sorority. Most will only accept a rec from an alumna. But some will take them from active sisters at other colleges. For instance, AFAIK Alpha Phi and PI Beta Phi will only take recs from alums. But Kappa Alpha Theta will take recs from active sisters at other colleges.

What I would do is to do the research on this in as far as you are able. Then you can go to your friend and say, "I see from your sorority's inter/national website that your GLO accepts letters of recommendation from active sisters. Would you be willing to write one for me?"

CU has not traditionally been a school that requires recs, but from the sheer numbers going through recruitment, it can't hurt.

Also, try friends' moms, church members, neighbors, teachers, camp counselors, coworkers, etc.