I agree with local nurseries but out here in Sacramento CA it's very cheep.
Just ask for "Green Moss" or maybee hanging pot moss. It comes in compress bales of various sizes and lossens up to the same consistency/texture as the reptile stuff except maybe longer pieces.
It runs arround $10 for a square foot or maybe slightly less size compressed bale and arround $14 for a 1-2 square foot compressed bale.
If you ask for sphagnum moss without describing the consistency or saying it is for hanging plants then you will 99% of the time be directed to the soil supplement sphagnum peat moss which is about the consistency of compressed powdered dirt. The peat is great for the bottom layer in a moss box hide for your tropical snakes but of no use for a corn or similar type snake.
Hope this was helpfull.
PS: the local nursery I normally get mine at is Capitol Nurseries
Sean