Don't have 250's or 300's in stock. Not much call for them in long shaft. I'll be placing an electronics order later this week so if you don't find them, hit me up...
why don't you just make your own if you have some 500k pots laying around, pretty easy to do, test the pot with a reader first to see it's actual value (sometimes 480, sometimes 502 etc), then solder a proper resistor between lugs 1-3, ground as normal and you have your pot custom.