I'm actually looking for the same thing. I'm about to do my first fluid change, including the motor oil, and I'd like to find something cheaper than what Polaris offers as well. On Amazon, the Polaris Factory ATV & Snowmobile 4-Cycle PS4 Plus Synthetic Oil Gallon 2876245 currently costs $43.99. I'm looking at the Liqui Moly ATV/UTV 4T 5w50 4-Liter which currently costs $36.70. Both shippers of these items offer free shipping.
I've read that Castrol 5w-50 is a good substitute as is Mobil One 5w-50 and Klotz ATV/UTV 0w-40, depending on your temperature range. These oil alternatives aren't too terribly less expensive though, so I'm not sure it's worth it to take the chance. I've read through some forums and I see a lot of, "I ran so-and-so oil and had good results with it." My response to that is, "What kind of results were you expecting?" Do these people think their ATV engines were going to seize up half way through their rides? What does "good results" mean? You wouldn't know the actual wear and tear results until after many a mile has been ridden and after a thorough engine breakdown and inspection, so take these types of comment with a grain of salt.
I think I've decided that I'll just go with the Polaris brand oil for all my changes - gearboxes and all. I'll just look for the best price I can for that. Right now, I think Amazon is the best way to go. It's costs $43.99 at Walmart and $47.99 on Ebay and I'm not even sure that's with free shipping. If I find out anything else, I'll update this thread.