check out below thread, and the reply i made to it. should help you a good amount.
http://www.tractorbynet.com/forums/wanted-ads/210646-tractor-tires.html
as far as type of tires as in ply ratings. do a search for "r1 r4 turf ply" many threads should come back, with what type of tire to get. granted for large ag tractor it will be r1's for field use. but the ply rating might be of interest.
as far as dealer or buy online, neither. ya need togo tire place, open the old paper phone book, not the internet phone books. and start calling tire places. again see link i posted above, and it should help ya find a few places and narrow things down. if it takes you 1 to 2 hours to call 30 places, then do that one hour. prices can vary from 600 bucks to 1600 bucks for the 555c TLB (tractor loader backhoe) rear tire. (that is a single tire) and it may require to call places up to 1.5 hour drive time away.
and to note it, let the tire place do the installation. and coming out and removing old and putting on new. to me, it really is not a DIY thing.
quick tip, if you do not want the old tires, tell the person that shows up, right off get go. and if they don't have a saw zaw, have one ready, so they can cut the old tires off real quick. vs jacking around trying to save the old tire for like a sand box.
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putting fluid in a tire, is more of a DIY thing, than taking off old tire an putting on a new tire. but if there is fluided in the tire, leave it to the guy that comes out. let them deal with the mess and pain of fluid in tire. ((they should have pumps, etc.. to deal with everything))