I've been leaving my backplate off for this very reason. Ebay appears to be your only real option in the EU. Search for "back cover (New Nintendo 3DS XL)". Looks like it will set you back around 11 euro.
Alternately, unless you have a special edition that you really like the graphics of, Google...
Loving this! Works extremely well. Any possibility of adding .crt support? On the PSP, these were handy because there was no 1541 emulation needed- instant loading of games (Google "EasyFlash scene").