Float tanks are AWESOME! I've seen build-your-own plans online before, so they are out there. Years ago I bought a used one, a really cool model that folded down to about 3 1/2 feet off the ground. You could then put a mattress on top and use it as a bed. Then push off the mattress, open it up like the top of a Volkswagen Westfalia Vanagon, open the top door, climb in and blast off. I'd sleep in there sometimes. Really deep sleep and you don't need as many hours as regular sleep. I bought it used and paid a pretty good price for it. Sold it years later to a friend who got a lot of use out of it as well. If you are in or near a major city look around for used flotation tanks. One of the most expensive parts is the pump and filter, which are crucial to any float tank running smoothly over time. You will also need to invest a fair amount of $ in 800 - 1000 pounds of Epsom Salts in order to make the water bouyant and soft. Heaters are fairly cheap. You want to set it up in a place where it can be permanent and untouched for a long time, cause if you ever need to drain it or move it you will need to purchase 800 - 1000 more pounds of Epsom Salts. Not fun and not cheap. They are a great tool for self exploration, very restful and relaxing and great for meditative states. I've enjoyed smoking cannabis for decades, but it was much too intense for me inside the tank. I felt anxious and way too much energy for that environment, so after trying it a few times in the tank I decided never again. Those experiences stopped me from trying anything else in there. I figured entheogenic experiences would be way too intense for me in that environment as well, so never tried them there. Altered States anyone? A nice natural setting is always better for me for those experiences IMO.
Anyway, that's my take on float tanks. If you have never tried one you should book a few hours in one at a health spa or other place that rents time in them to see if you like it and get the feel for the experience. Owning one is a commitment. You need to filter the water often, always keep the water heated properly, they take up a lot of space, and you want a shower or hose nearby, cause once you get out you are covered in saline water that will burn your eyes if you rub them and will crust over on your skin if you do not wash it off quickly. If you do want to own one, you can build your own, buy a pump and filtration system and the 800 - 1000 pounds of epsom salts, or you can look around for a used one. A lot of health spas and colonic facilities tend to also have float tanks, so maybe call around there to find a used one. Good luck.