How can I get more keyboard software instruments for GarageBand? I've been using GarageBand to play my MIDI keyboard for a few years, just because of its built-in software instruments for keyboards. Now I'm thinking about purchasing some jam packs only for the extra keyboard software instruments.

Which ones should I buy? Are there any more places or third-party software where I can get more keyboard instruments? I really want to have a huge selection of keyboard sounds, but don't know where or how to get them.
There are many third-party virtual keyboard instruments. Do a search at the kvraudio.com virtual instrument news site. Anything that is in Audio Unit (AU) format will work with GarageBand. Here is a search result I have prepared for you.

Popular products you might consider:

Anything by Native Instruments

Pianoteq

Buying Apple Logic Express or Logic Studio will give you all the Apple Jam Pack sounds in one purchase, with many other features besides. Apple Logic Express and Logic Studio.

Finally, read Keyboard Magazine. They provide a lot of coverage and reviews of products in this category.