Hey there! I pmed you a few days back...
If you are doing a back carry, then you want to go for the gauze-
I'm pretty sure I've seen it there before--- thing gauze like you wrap wounds with- loose weave, but not quite as loose as a bandage. Think breezy summer curtain fabric. Think hippy skirt fabric...well, you get the idea. It has to have some give to it, to cradle and support the baby. Muslin is just too tightly woven to really conform to the babies (or your) body. It should be thin enough to see the lights of the store when you hold it up, but not so thin that it wouldn't support your baby's weight. I know it's a little overwhelming, but my best advice is just to try and find something close, and try it out around the house to get a feel for it. I have used cotton knit (tshirt material) wraps for my baby until she was about a year, but they stretch, so I'd be careful for back carries. I tried the rucksack carry with the knit and it was just so stretchy that it felt like she was gonna pop out the bottom! But, cotton knit is a very forgiving wrap material to learn with, so it may be worth it to you to try it out and then give it to a mum with a littler one. Also, have you seen if there are any NINO or babywearing groups in your area? There was a mom selling crinkle cotton on TP- maybe she has a pic you could look at. Good luck!
and I also read on here that stretchy fabric doesn't work for heavier (or older) babies.
and no, there aren't. there's milkworks but I am not sure I want to pay their prices for a wrap. ($40+)