I tried many too before I found a fit. Ended up using these black nylon ones 6mm x 20mm (same as Mark's sizing). Got them at either AutoZone of Checkers/O'Reillys.....I think it was AutoZone.
If you need these just shout, they're an extra set of 4 I bought once I found that they fit.
Pics won't let me attach my new pic for some reason, so you can see it on the home page pics, yellow package.