Memory Mappers some times appear on ebay, although they seem to be quite rare!
The forthcoming OSS fair should be good. Sunrise may have some news on the new MSX Ethernetcard, which I beleive may include a 4Mb memory mapper, at least according to the leaflet on there website
I'm in the same situation as the OP. I've found someone in Spain who will expand my WX to 256kB for 39 euros and 512kB for 54. I'm tempted to go for the latter. Does anyone have an opinion about what a fair price would be?
Talk to Hans then, not Bas. Tell him what Nyyrikki said and there shouldn't be any problems upgrading it to at least 256kB.
You could even do it yourself... Just see what kind of RAM chips are already present in the computer (amount of pins and kb/chip), and find similar RAM chips, then plug the new chips into the empty sockets. Or solder them in, if there are no sockets.