Locating a hidden batch of NOS of the desired EPROM version seems highly unlikely. Did you check with Gary Kammerer who supports Cambridge products?

If not the only choice seems to be to find someone with an L-NAV with V5.8 and;

1) Buy it.
2) Borrow it, remove the EPROM, and find out how to make copies of it. Not really a do-it-yourself kind of activity but there are services on the web that will make the copies for you. FYI when shipping the EPROM wrap it in tin foil to protect it from static damage.

Best of luck.