Rim strips (a.k.a. rim tape) are pretty forgiving; you can definitely use the "regular" 26" size (get the narrow ones (12-16mm) if you can, though it doesn't make a huge difference). But it shouldn't be too hard to find some in 26 x 1 3/8; here are some in the US; if you are indeed in Belgium, I'm sure you can find an EU source.