There are a few instruments in different JamPacks. What kind of instruments are you looking for?
The JamPacks are no longer sold separately. If you want to buy them, the best bargain would be to purchase MainStage. It installs all five JamPacks and GarageBand can use the installed sounds.
I found these instruments that can play chords, mostly guitars:
Jam Pack 1 has the nice 12 String Guitar:
12 String Chords: Each note plays a complete major or minor chord, with adjacent octa- ves playing up and down strums.
- Mod wheel adds vibrato.
- Higher velocities increase loudness and attack.
- Notes E0 (Error: E1) through D♯2 play major chords. Notes E2 through D♯4 play minor chords.
Jam Pack 3: Rhythm Section Instruments:
Acoustic Harmonics, Flutter Harmonics: Acoustic guitar harmonics. • Mod wheel adds sustain.
• Highest velocity plays tap with harmonics. Bluegrass Banjo, Atlantis Banjo: Traditional plucked bluegrass banjo.
• Mod wheel adds vibrato. • Low velocities alternate between stopped and muted note. • Higher velocities sound more open. • Highest velocity adds slides up to note. • Notes below E0 play chordal rolls, string slides.
Dobro Chords: Each note plays a chord or interval, some with slides up to the chord.
• Notes E0 to A♯ 3 play major chords. • Notes B3 to E5 play minor chords. • Notes above E5 play open fourths. • Mod wheel adds velocity.
• Velocity increases loudness and intensity.
Eighties Power Chords:
Each note plays a complete power chord or power strum.
• Notes B0 through A2 play power chords; • notes B2 through A4 play stopped (muted) chords. • Mod wheel adds vibrato. • Higher velocities add intensity.
Sunburst Power Chords, Mosh Pit:
Each note plays a complete power chord or power strum.
• Notes B0 through A2 play power chord. • Notes B2 through A4 play stopped (muted) chords.
• Mod wheel adds vibrato. • Higher velocities add intensity.
Twangy Strumming: Each note plays a complete major or minor chord, with adjacent octaves playing up and down strums.
• Notes E2 through D♯4 play major chords; • notes E4 through D♯6 play minor chords. • Mod wheel adds vibrato. • Higher velocities increase intensity.
Jam Pack: World Music Instruments
Spanish Flamenco Guitar:
• Mod wheel adds harmonics an octave above the note
played.
- Velocity increases attack and volume; changes strummed chords on notes below B1.
- Highest velocity adds slide up to note.
- Notes below B1 (Bug: Chords only below C1, harmonics between C1 and B1, major chords below C1) play strummed chords; notes above B5 play body taps and knocks.