The Fox Racing Shox dust wiper kit are available in 32 and 36mm. You should be replacing these every year or so. To be honest, Fox recommends changing your oil every 40-50 hours, and then your seals every 2nd or 3rd time.
We replace these just about every time we overhaul a fork, because most people wait much longer than 40-50 hours of use...
As soon as you get dirt in them, they start destroying your stanchions. So if you replace them every year or so, your fork will be much happier.