WSP are available at some hardware stores (Lowe’s, Home Depot), agriculture-type stores (Tractor Supply Co.), and I’ve even seen them at bulk chains (Sam’s Club, BJ’s, Costco). Call around and see if any of your local stores carry them. They are typically available in 40 pound bags, at an average cost of $6 per bag. So they are a very, very cheap litter alternative to overpriced petstore litters.
They are only available seasonally, so now would be a good time to start looking for them! If your local stores don’t carry them, check to see if other locations a bit farther away do. In my case, our local hardware store does not carry them (we are in a suburban area), but by just driving about 20 mins away to a more “rural” area, I can purchase them. They are definitely worth the drive!!! We go once a year and buy about 8 bags (for 2 buns) and this lasts us until the following year.
Horse stall bedding is similar and also safe to use as litter. With WSP, be sure they don’t have added accelerant.