Where can I get specific accessories for my Canon “Selphy” photo printer?

I’m looking for the Paper Cassette PCPL-CP400. It’s an accessory for the Canon Selphy cp910 that allows me to print pictures that are a different size than the standard postcard. I’ve looked all over (stores and the internet) and only found some on Amazon/Ebay that were outrageously priced. It shouldn’t be priced over $25-30
I’d prefer it to be from within the United States, but if I have to get it out of the country I at least want it to be a safe, secure, and reliable website. Any suggestions?