It's not super cheap, but I use coco coir. It comes in bricks you expand in water, and my Russian loves to burrow in it. You can mix it with cheap organic soil, which is what I do. Seems to hold together better.
What I like here for the indoor enclosures....word of advice in the event you do use it;
It is fluffy to begin with, so when you put it into the enclosure you will need to dampen it a little (pouring water in slowly the first time as it absorbs alot of water so it takes a bit, but there after it takes the water quickly)
Then after the water has been taken into the substrate, then with your hand press it down so that you firm up the surface that the tort will walk on so they can get firm footing and traction
Make sure that you make the substrate at least 4 inches deep, even deeper is better---this substrate is good at retaining the moisture and then as it dries from the top layer of the substrate it is still moist under the top layer, which is nice....
Okay, so wait round for the others to also give you their choices...fun stuff...I would not use sand, some do, however to me the increase in the risk for impaction is just not worth it...again, in my opinion...
I am not sure where you are from. But, I go to my local orchard supply in California and buy a huge block for 13. It has made my 100 gallon tank 4 inches deep.