Windowed cap source?

[nathanscribe]

After trawling a few component suppliers here in the UK, to no avail, does anyone know of a place that stocks single-unit windowed MX-compatible caps? Profile is not an enormous issue as this is for custom projects, not an existing keyboard - but preferably something reasonably generic, no legends, with a window that suits LED placement on regular MX switches.

Any suggestions appreciated. Happy 2014!