Two t-shirt quilt books carried in my local quilt shop recommend fusible tricot interfacing: So Sheer or Fusi-Knit, although, a woman in my neighborhood who has a business making t-shirt quilts told me she uses whatever interfacing/stabilizer is available at Jo-Anne's that's cheap and abundant. Go figure....
It's frustrating when you want to try something new and there are so many different recommendations about how to get the job done. This is one of the snags that's holding me back from starting a t-shirt quilt. If this was fabric, and I messed it up, I could go find more fabric. But with a t-shirt quilt, you only have one shot at getting it right--that is, if you ruin a t-shirt, you can't get it back. Sigh....