I use a marine bluesea one. Works great. I believe its 350a continuous which is plenty.
I struggled to find a 500a continuous switch and the size of wiring required was massive; and bigger than the wire supplied with the winch! Remember that the winch will only be pulling its rated load when its at absolute full chat which wont be for very long. The blue sea one i use is rated for 1000a upto 1 minute.