How can I get more keyboard software instruments for GarageBand?
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.