Garage band is severely limited, though its brilliantly produced little loops all 'fit together' no matter what strings of them you pull together.
You will be wanting / needing a desktop recorder - midi, (Cubase or similar, a basic version should do you) and some independent samples which can be played through it. [ADD: both Finale and Sibelius, software to produce notated scores, have built-in midi which will also function well for you.]
There may be quite a number of free (and safe to download) individual sound samples or libraries.
For any references or recommendations for those I think it would best to ask this same question in the pop music section, because more there are likely to know, and to know specifics to which they can give you references or direct links.
You may also be able to find, say via Craigslist, an inexpensive but good older and second-hand Proteus (or similar) sound-sample / synthesizer. Depending on the model, there are various sound palettes in each. You will need a desktop recorder and midi interface to use it.
Best regards.
Purchasing a sound library, and the