I got my ivermectin from the local Farm & Fleet. See if you have a similar store nearby. Also, ask your local pharmacy for a syringe that has a removable needle. If you tell them what it is for, they will often just give it to you free of charge.
I don't think I have a store that would have ivermectin nearby. Anything here that says "feed store" is really just a chicken store (it's Los Angeles, a LOT of people have chickens) and wouldn't have ivermectin, most likely.
Thanks for the instructions on ivermectin dosing. I'll need to explain it to his owner and she'll be doing the last two treatments, so I want to make sure she understands.
Thank you so much for all the help dealing with this! Hopefully there won't be any more issues with mites in this house!
Speaking of seeing them, Pan Pan looks much better. His skin is healing and there's no more itching. He's starting to get some fur back, too. Thank you to everyone who helped me figure this out (especially Lynx and Bpatters!).