In the Netherlands I've found the 3-tab (left-middle-right) folders, made by Atlanta and SMEAD.
They both are the types Supertab and both have the same article numbers (same product, other brandname stamped on it?).
A6002-6437 - Smead SuperTab (size A4)
For The Netherlands they could be found f.e. at www.discountoffice.nl, but any shop carrying Atlanta and/or SMEAD products should be able to deliver them. In other countries you can search for shops who carry these brands, perhaps they can deliver these articles.
Prices ranges tremendously. Discountoffice.nl was the cheapest in my search.