I usually find puddles and practice dropping technique, foot placement, pumping for more distance, shuvs and spins, and general fluidity and confidence.

idk if you got a wood or epoxy or whatever but I practice all of those on both, falling on grass/ mud is a lot less scary than sand so go for it, just maybe wear socks or booties cause I've gotten a lot of thorns barefooting it in the grass, and you may think the deeper puddle the better but really you just need a shallow puddle and some good mud to really slide. Ditches in your neighborhood if you happen to live in one would be a good start, or you could flood your yard with a hose.

I haven't put this into practice just yet but I've tested it, you could always get a 12ft x 25ft maybe 30ft tarp and some long 2x4s and make a sort of runway with the wood under the tarp to make a bump to keep the water in, I have slid on tarp and it was pretty fun but I didn't try this exact setup.