British Encoders ( encoder.co.uk )will be able to supply what you need, but they won't be that cheap.

I think I've still got got an encoder kicking around somewhere, that I bought from Zapp a few years ago.
Just a had quick email search for the order, and the part no is ISC3806-001G-1024BZ1-5L, and it's 1024 line, so 4096cpr.
If you're interested, I'll have a rummage in the workshop tomorrow night, as I have no idea where it would have ended up during last years major workshop rearrange.