I would use a "T-shirt" knit, which I believe is an interlock knit.

Here is a link that might be useful: Jo-Ann fabrics online

T-shirt knit here as well.It will probably cost you more to make them than it is to buy them.Epecially when you factor in your sewing ,shopping,and prep time.Why not just buy plain ones and decorate with paint pens,or embroidery.Just a suggestion.
Kathi

Thanks,
Yes, found the fabrics and they sure are expensive. Guess I've been away from sewing for too long. Oh my gosh! I see that there must be a big interest in organic fabrics.

Remember when it was cheaper to make your own things, but then the styles for young girls that I'm finding on the racks are just so ugly and immodest.

Thanks again

I've found quite a bit of knit fabric at thrift stores. If you have a Mill Ends Textiles anywhere near you, that's a great place for knits, too. The nearest one to me is 1 1/2 hours away, but in the same town as one of DH's doctors. I check their sales flyers online when he has appointments.