I myself would not buy anything, or if it comes to merchandise then I will rather create it on my own as "fan work".
Believe it or not, some works require just basic printing for being a "merchandise". You can order a tee shirt with your fave print at a printing workshop. You can do so with copy books, pens/pencils, playing card sets, bags, wallets.... whatever needs basic 2D printing.
Recently, 3D printing is starting going to the masses. Therefore, if you have a 3D model of that girl that appears sometimes when you play osu!, you would be able to definitely print it on a 3D printer, or for some experts - make an action figurine or a doll. There can be 3D printing workshops too.