The JamPack World Music has been sold separately on installer DVDs and is no longer sold. It has been quite expensive. But if you purchase MainStage 3, all five jamPacks will be installed as part of the bundle. And they will appear in GarageBand and Logic in the Legacy section of the library. There are two Accordions in the Jam Pack World Music, Tango Accordion and Polka Accordion, plus some nice accordion loops.
When you download all sounds for MainStage and install them, they will appear automatically in GarageBands Sound Library. MainStage will install a lot of sounds. You may want to be selective, when downloading the MainStage sounds. They require a lot of storage.
Once you installed MainStage, go to the menu MainStage 3 > Sound library > Open Sound Library Manager.
You will now see a list of all available sounds. Open the "Legacy" section to find the Jam Packs. I would primarily install the Jam Packs.
After you downloaded the loops and instruments, go to the Loops panel in GarageBand and scroll down to "Reindex all Loops". Then restart the Mac to update the sound library. Now GarageBand should be seeing the additional instruments. You can use the JamPacks directly from the Library panel in GarageBand. Just click the Legacy section in the sound library, when you select the patch for a software instrument track. And the loops should appear in the Loop browser.
