You can edit or add a Buy link to let your fans purchase the track from another site (such as iTunes, BandCamp, etc.) through your track’s edit page in the ‘Metadata’ tab. You can access your track's edit page at any time through the pencil icon below it's waveform. 

Once you click 'Save', a ‘Buy’ button will then be rendered below the waveform. 

You can change the wording of the Buy button so it doesn't say 'Buy'. This feature is available for Pro and Next Pro subscriptions. You'll be able to add a total of 22 characters to the ‘Link title’.  This is a great option if you wish to direct your listeners to a special link, i.e. a donation link.