Cloth-backed vinyl, commonly known as “oilcloth” though that term also refers to much older versions of water-resistant cloth, is probably your best bet for a cheap set of waders. It’s commonly used for furniture, athletic mats and equipment padding, The material’s waterproof (remember to seal all seams), durable, fairly inexpensive and highly available.
As a garment, it’s going to be somewhat less long-lived than materials more commonly used in commercial dry waders, as the material will eventually harden or wear and split, especially if left exposed to the elements. It can be patched fairly easily, but the patch makes that area stiffer, so if it occurs along a natural “fold line” like the back of the knee, it’ll reduce mobility. You’ll also need to come up with something durable and non-skid for the soles of the waders; I’d recommend making a “sock” of the waterproof material and then just slipping on a pair of aqua socks for traction.
Most commercial waders I’m familiar with are made of a laminated neoprene. Neoprene is a closed-cell foam very commonly used in outdoor garments intended to get very wet, as the material is highly insulating with its thousands of enclosed air pockets, which helps keep that 36* glacier-fed creek you’re trout-fishing in from giving you hypothermia without a drop of water getting inside the waders. For the same reason, however, it’s less useful in warmer weather, and laminated cotton duck, canvas or denim is more common for warm-weather waders.