Almost anything with a high cotton content! Old t-shirts, flannel shirts, flannel pj bottoms, knit jammies, even cotton sweat socks, lol. Towels, washcloths, and dishtowels. You can even use woven fabrics from dresses or slacks on the outside of the diapers for a cute print. If you have soft worn in old khaki pants they're usually twill & can be used too.
Anything soft & absorbent can be layered together for a soaker or a doubler. Flannels sometimes wear so that they feel more like a super soft twill than a fuzzy flannel & those are great too.
Recycled towels can fray pretty badly if you don't take care to fold the raw edges inward & stitch securely. The fraying threads ball together inside your diapers to form diaper rocks & the terry can get stiff so use only 2 or 3 layers of an old bath towel & make a lay in doubler to beef up absorbency. The less layers in the soaker pad will help reduce stiffness. I like recycling washcloths for this reason b/c the edges are already serged. No diaper rocks & some of my oldest first diapers I ever made haven't had any problems. I even bought a big pack of cheap washcloths from Wal-Fart (sp mistake intended
) & those worked really well folded into thirds.
HTH, have fun