My MDs probably wouldn't be much use to you because all my mods have been re-modified so at the very least you'd have to adjust the gearing all the time.
Sometimes, mods have a set of MD files included. TomMKs 93 mod has a very comprehensive set IIRC (MDs for many tracks are included).
Also, I think there are a lot of MDs some of the trackpacks. In addition, many track makers include an MD with their circuits.
Only a few circuits don't have an MD. Other tracks don't include a separate MD file, but the data is there, it just needs to be extracted from the .DAT. When I get a new track I always load the .DAT into CMagic4 and save the default MD in the circuit folder. (Another benefit of doing this is it will update the titles/comments with the ones from your customized cMagic4.ini). It only takes a few seconds and it's always good to have a backup.
Very occasionally, when there is no MD, I copy the MD from a similar circuit, ie slow and twisty or fast and free-flowing etc., and use that. You can get errors doing things that way (like cars crashing when entering the pits), but you'll learn a lot by fixing them. Be aware of bumps however. If the donated MD puts a bump further along the track than the number of sectors the track actually has, it'll crash.
Hope that helps.