I highly recommend you invest in getting a MIDI keyboard if you’re making music on your computer.
Clicking on a mouse alone to make music slows down the creative process. Once you start playing on a keyboard you will find it easier to play chords, play out melodies you have in your head, and focus less on the computer and more on the music.
I use sites like Craigslist.org to buy and trade used music equipment.
Also check out 5 MIDI Controller Keyboards Under $100.