Print your own band-themed thank you notes from your own printer. These were designed one evening in one of my Twitch streams (follow marchingbandimals on Twitch to see more). I'm making them available to you FOR FREE from my shop. You do still have to go through the check-out process, but I guarantee, they are 100% free!
Just print them out on some thick paper or card stock, cut them apart and fold them. You can purchase quarter-fold envelopes at most craft/hobby stores that will fit these cards perfectly.
These require a PDF reader (such as Adobe Acrobat) to open.