Where can I buy rivets for an aluminum boat?

You should be able to get them at places like west marine. I'm surea marina or boat dealer could direct you.
If you can see the crack, and have room to work, look into a good "thin gauge welder". Once again a reputable marina or dealer will know someone. (95% of the time the weld will be stronger than the surrounding metal.

I've been told that boat rivets are the same as the rivets used on semi trailers. Check into truck shops, I'd bet they are a lot cheaper than something "marine".

Try a fastener supply place (Suburban Bolt, Metrics Unlimited) or a machine shop supply (McMaster Carr, Graingers) and buy in bulk (at least 100 packs) for EXTREME discounts over buying them from a marina. If you only need two or twelve a marina is not bad....