That sounds like an exceptional belt life to me! It is worth your while to have a spare around, just because of the breakage issue. They are generally cheap enough. If it is a conventional "V" belt, any decent hardware store will have them. The size is printed on the back, or if you have the broken one, you can match it to what they have in stock. Buy 2.
As to getting them on and off, it depends on the mower design. On an old Sears I had, many moons ago, I had to drop the mower from the tractor (a couple of clevis pins), turn the mower unit over, and then loosen some tensioners to replace the belt. A good set of socket wrenches were all that I needed. The bolts were on pretty tight, so don't even think of using a crescent wrench!
The manual that came with the mower likely shows a good diagram, and maybe even directions for servicing the belts. You might try a Google search to see what you can find, or check their web site.
As long as the engine and transmission are in good order, a broken belt is just part of the cost of doing business. I never could get more than a couple of seasons out of a set of belts. Your milage will vary...