The best place to get them is either a local electronics distributor (not radio shack) or online such as farnell or mouser

Theres a tiny amount of debate of which type to buy that pops up once in a while, some say the original electrolytic can types are better, to make a 1:1 replacement because of their behavior in certain situations, most around here will strongly advise the use Tantalum types.

if you use Tant's you will need more because they don't come in the same values as electrolytic can types (so you have to stack them up) and they cost more, most people say they wont ever leak, which is FALSE, but they have much much less chance of doing so, your going to have to practically blow them up first

If you use electrolytic can types (i keep calling them can types because Tantalum's are electrolytic also) they will be cheaper and easier to deal with, and probably will perform better in certain areas (like the power supply) but your going to have to replace them again in another 10-20-30 years

PS: if your new to soldering, save your mac, goto a thrift store and get some old looking VCR or something to practice on first, it does take some getting used to