I think that the Walker Evans and Elka brands are the most popular, as well as two of the few companies that make shocks for the P500. I've read good about both on this site from members that have purchased both. Basically I think Walker Evans has 1 or 2 models to chose from and Elka has 5 (Stage 1 through 5) so it comes down to how much money you want to spend vs. how many adjustments you want to play with/deal with. Plan on about $1600 a set average.
When I get around to it, I will probably buy Elka's for my P500 for I have Elka's on my TRX250R and I am completely happy with the performance and durability of them (had them on my machine since 2009). No breakage, leaks or stuff wearing out.
I cannot remember, but I think there is 1 or 2 other company's out there that make a replacement shock for the P500 that are not adjustable and priced lower, but I cannot come up with the names right now.