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...