Well sifted and washed play sand is a semi decent loose substrate to use. Resembles a part of their natural habitat at least in some way and is what you are asking about.
Before I go on, I realize you might already know this, but I feel I should say it anyway. If you are going to use loose substrate, regular full substrate replacement will be MANDATORY in the enclosure that you are using it. So that means about once a month or so you will have to change out the bedding entirely, this is to minimize the risk of bacteria growth in the enclosure...which of course would lead to further health problems for the reptile.
Also, there is something called Sani-Chip; which is a blend of beech, birch, maple and very little aspen cut to a small square-like shape and heat treated. I use it for some of my reptiles and it works very well, doesn't get stuck in their stomach, the reptiles are comfortable on the substrate and it passes through their digestive system very easily. A lot of breeders use sanichip as well.
I usually pick mine up from a reptile show in PA here for about 20 bucks for a 30 lb bag (not sure the amount tbh) I believe online is around that price as well.
There is talk of using alfalfa meal, but I find that it is very dusty and holds moisture way too much. When I changed out the bedding it was very hard and clumpy (luckily no mold) so long story short I learned my lesson and will never use that again.
Straight shredded aspen (the kind for hamsters and whatnot) is not a very good choice due to the toxins it gives off because it is not treated properly to withstand the high temps of basking lights for reptiles! please don't use that kind of loose substrate
*if you are going to use playsand* please sift it very well and then wash it very very very well and let it bake in the sun outside for a day or two (24 hours, not just 8 hours while you are at work).
other than that there really isn't many other options that I have personally heard of for loose substrate. You will get people that will preach using tile and whatnot, while it is safer, I know some people who get grossed out by wiping the stool off by hand using baby wipes and some that just don't want to cut the tile to size or get it cut(i've heard this excuse). I personally use loose substrate because my bearded dragon seems to be a LOT more comfy on it then tile (it really stressed him out for some reason, I tried everything).
Best of luck! I've tried every substrate on the market to my knowledge so if you have any questions please ask!