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If I understand correctly this is for the transmitted light/brightfield  incandescent lamp. 

A cheap regulated lab power supply with the right output range (0-12V) works fine. 
I have several Leica Ortholux I scopes (the black ones from 1960's) where I replaced the original Leica plug with two "banana" plugs (is that a right term?) that go into a 0-12V lab power supply.

Another option is to look for car/RV light dimmers, but you would probably have to feed it with 12 V DC.

Similarly, a cheap (~$3 off Ebay)  dimmable 0-12V, 8A dimmer switch should work if you can supply a DC power to it, e.g., from an old  laptop computer power adapter.
 
https://www.amazon.com/LEDwholesalers-LED-Strip-Lights-Controller/dp/B003L4KKF2/ref=pd_sim_60_2?_encoding=UTF8&psc=1&refRID=4YTD01ZPNVBFTPV2E0H5 


Best Regards,

Stan Vitha

Microscopy and Imaging Center
Texas A&M University