Linear encoders to fit optitracker interface?

Hi and thanks in advance...

I have an Optitracker Quadrature Encoder interface from
MicroKinetics. They offer only rotary encoders for the unit. This
interface requires even resolution (they use 1000 line encoders with
it), two channels, and an index signal. I am on the learning curve
here, so forgive the lack of detail. The spec sheet indicates it will
accept input from either incremental or quadrature encoders,
mutipliers to 4x. Eight TTL compatible inputs available.

Can anyone recommend a source for linear encoders that will work
with this interface? Ballpark cost?

Also, has anyone got a good deal on a P266 class motherboard/computer
with 2 ISA slots?

Thanks,
Joel