you can use Regular old Hydraulic fluid if you're not picky. On my last couple trans rebuilds, I put in one quart of pure synthetic lucas engine oil additive, and 2 quarts of John Deere J-20 spec hydraulic oil. should be available at most parts stores or at tractor/farm/ranch supply stores. Buying it from John Deere directly is too expencive. You can buy a bucket of Harvest king oil (5 gal) for about $60, and have enough oil to change your transmission 6 times!
BTW, you may have to jack up the rear of your machine to get 3 quarts in. the extra quart will help keep the trans lubed. it may puke some oil out of the breather on a hard run, but it won't hurt anything. I think polaris's fill spec is 1.5 quarts. they'll take 3 quarts just fine.