i agree with the pp, my fave sbp's are ones made from woven wraps. she sometimes has those available on her website, and as the pp said you can get them very reasonably gently used as well.
Jan is also now redoing shoulders on the old Maya Wraps and she's only charging $25! that fabric isn't as awesome as a german woven wrap, but it's a pretty nice lightweight woven fabric and you can't beat the price!
short of that, i'd choose a linen or linen blend fabric, i think those make very nice ring slings.