What is is about the 14mm that isn't found in the 17mm?
Try looking on Ebay for "14mm sump washer".
I find them useful for a lot of things flashlighty.
P
If you are looking for 1-off, just sand it down to 17mm diameter. If you need a bunch of them, I have a lathe...
Just a bit of dremel work to open it out to 14mm.
P