I've been using one of the organic topsoils sold in bags from Lowe's. Make sure there's no fertilizers or anything else mixed in. It's about $2.50 a bag and you can grow plants in it if your habitat has room. Maybe I'm making this too simple, but tortoises live in dirt in the wild, not some fancy store-bought substrate. Its easy and cheap to replace as it gets dirty, holds moisture well, and is most similar to their natural environment.
For a baby, juvenile or adult.? I personally use and like cypress mulch for juvenile/adults - holds moisture, doesn't mildew, cheap for a 2-3 cubic ft bag. Easily available (in Season).