Those sold in your local pool stores, size varying depending on size of pool, and whether it is an above ground or inground.
There are however other options, like elastics and safety covers. Those tend to be somewhat more costly (the later going into the 1-2 grands). They offer unique benefits too. Elastics makes a net under your winter cover and gives additionnal support. They shouldn't need adjustments like water tubes does, and arent any more troublesome.
Safety covers, like Latham Splash Canada's (www.technican.ca), may offer additionnal protection. Their brochures and ressellers shows images of people standing (or lying) on their pool covers... They sell a feeling of safety, and they can look pretty neat, are engineered for your pool's dimensions, but are they REALLY worth their price?