Fish Merch
Now I will admit, it IS kinda strange to have a page dedicated to listing sites to get RBF merch but I needed to pad out my site with a few more pages and anything that can be made quickly and easily is highly appreciated.
Plus I might prevent at least 1 person from buying awful bootleg merch off redbubble or something by directing them towards more reliable sites (The print quality is always SO bad with bootleg merch off redbubble!)
The Fish Market
Nothing's more reliable than the official merch site! Here you'll find shirts, stickers, pins, Physical music releases (at least when those are in stock...), posters, ect. Plus you'll be directly supporting the band because they're hot and cool.
Discogs
Discogs is kinda like eBay but exclusivley for music. It also acts like a music database, showing you pretty much everything your favorite band has ever put out. (Including releases that AREN'T on spotify!) I've gotten a few things from here myself and I HIGHLY recommend it.
eBay
Now it's no secret that eBay is a terrifying place, but it's also the place most people go to when it comes to finding old band merch. This is pretty much the only place where you can find vintage RBF merch, and if you're lucky you might even find something in your size!
When it comes to shopping on eBay, It's all about patience and observation. (AKA waiting for something cool to go up for sale for a decent price and making sure to check the condition of the item.)