Either buy a bundle (#2 white cedar, and pick out the better ones) and cut them down, or visit a jobsite with shingling in progress and ask for offcuts (bring coffee and doughnuts to improve your odds of "yeah, sure, take whatever you need from the scrap pile")
A quick glance shows that $10 will get a bundle of "utility" grade shingles, which is codeword for "bad" but you can probably trim those to get enough usable shingles for your small job and a lot of kindling for lighting fires with.
Any job will have a large number of cutoffs where the top of the wall or roof is finished with short shingles - no short shingles are sold, so all the 4"-8" shingles lining the top have a corresponding 12"-8" cutoff.
You might think you'll just buy a pack of shims, but shims are too narrow to function as shingles (anything less than 3" wide is not wide enough.) That might even be what was originally done on your bird feeder, upon a quick count - and part of why it failed so badly. You need at least 1-1/2" from a joint in one layer to a joint in the next layer - thus, 3" is a minimum.