I ask the tires shops for weight, I have walked out with a couple pounds of weight. I ask for their broken strips etc, they wont give you full strips, bring them a couple cheap pizzas and they will keep you well stocked.
Harbor Freight sells 2 - 2 1/2 oz strips for 4.98.
use lead and melt it down into an altoids can or something small like that, or thin sheets of lead, then cut the sheets into strips. That way you can make your own strips of lead wheel weights, works just as good as the ones you pay for. if you have lead laying around, what do you have to lose?
to do a rough balance (really jsut to check and see), get the kart onto a level stand and spin the wheels on the front spindles....it works well enough to show us if the wheel is bent or severly out of balance...and only takes a few minutes....got some plans on the drawing board for a friend to weld me up a stand to mount a front spindle on to do jsut this....
Balancing also helps keep the tire temps more consistent, helps on tire wear, cuts down on bearing wear, and gives the driver a better feel at the wheel. As said above, for short track racing it might not be much of an advantage, but for bigger momentum or high speed tracks it sure does help.
Also, be sure to balance AFTER you cut, finish, or internally prep your tires.
FWIW: We stock and sell kart tire balancers and lead stick-on weights.