If you mean a 1960s unit, your best bet would be Phil Smith at www.scaleslotcarspares.com (in England).
The big block-shaped MRRC guides may be scarce nowadays but Phil has a vast store of parts. Much of it is not listed on the website but you can contact him through it and ask.
The guides for the later 1960s Airfix-MRRC units are completely different and more common. If it's one of them that you need, specify which one - probably Clubman Special but possibly ballrace chassis.
As Rob says depends on which one you want, a picture of the unit you have would be helpful as there are several different types, if it is a Ballrace unit then these are unobtanium, I have the others including 4WD (these are expensive
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