You should be able to find additional loops online, both free and not so. MacLoops.com and Looperman.com are two that I've come across. You could also search for backing tracks as opposed to loops if you want something to jam along with.
Adding audio sample to GarageBand
Connect your iOS device to iTunes, select it in the left-hand column, open the Apps tab, scroll down to File Sharing, select GarageBand, add files with the button or drag and drop onto the file area. Files should be uncompressed audio in AIFF, WAV, CAF or Apple Loops format at 16bit, 44.1 kHz.
Insert audio sample into song
Open a song, switch to the tracks layout if necessary, then tap on the loop icon, click the Audio Files tab, then tap & hold on a file to drag it into position on the tracks layout.