when i did my 500 i got the parts at a local sled shop. they worked on everything and were a dealer for aftermarket parts so it was only like $15-$20 a bearing. if i remember right the bearing on the chaincase side is a sealed bearing and the clutch side is a sealed w/a sleave and set screws. i would say that 95% of the auto parts stores in my area have the bearing cause they are a pretty standard size. also, when you have it out, make sure the shaft itself isn't damaged. mine was, and thats more work.
since you said you changed the chain i am assuming you know how to drain the chaincase and remove the gears. do that. the chaincase bearing may/maynot have a snap ring retainer, i don't remember. but if it does its visible from the chaincase side. other than that i should pop out w/ some taping. i found it easier to remove the whole chain case (3 bolts after the gears are out) and then change both gears. the cluth side: remove the belt and the secondary clutch. its a single 1/2" head bolt but make sure you get everything and remember the order it goes in. bolt, washers, spacers, clutch/key, and then the rear shims. then the bearing should be in a cup, remove it and the losen the 2set screws on the bearing sleave. that should do it. if you have more questions, ask. i did this on my sled lastyear and my new xcr this week-end, so its fresh in the memory.