Iflash.xyz have everything you need for iPod classics and iPod minis, the mini is just my fav, I had one when I was a kid and now I have another (well, 3) now I'm in my 30s
just do it yourself, it's super easy, only takes a cheap adapter you can get on ebay or from iflash and like 15 minutes of time, i used this guide to put a spare 32gb in a mini i bought and it was reaaaaally easy https://www.youtube.com/watch?v=1GY761U4vik
that's a mini, they use tiny microdrives inside, they're tiny HDDs in the form factor of a compactflash card, you can very easily replace them with a compactflash to SD adapter and put a shit ton of storage in them