Medieval French is referred to as Old French and is a collection of dialects for roughly the northern half of medieval France, much like Old Occitan (also known as Old Provençal) is a collection of dialects in southern medieval France. You may find that adjustment helps your search online and in libraries.

Google Books is great for resources like this, because they've scanned a lot of core 19th century academic works and made them available in complete form. So with a quick search you get volumes like this:

Robert Kelham, A Dictionary of the Norman Or Old French Language: Collected from Such...

There are also a number of online databases like Uni Texas Old French lists.

Most dictionaries of medieval languages show 'base forms' which means you'll need to understand a bit about conjugations and grammar to make sense of things. Google books has material still in copyright like E. Einhorn, Old French: A Concise Handbook and so it's incomplete but still useful on that.

Finally, remember that medieval vulgar languages were not codified and taught with print dictionaries at the time so we find a lot of variance in spelling in the sources; even medieval Latin which was largely codified suffered from this at times. It makes translation often very slow.

Try: http://www.lexilogos.com/francais_dictionnaire_ancien.htm

Or

http://www.micmap.org/dicfro/introduction/dmf

I think the first link is the better one.

For the second part of your question, maybe try /r/translation.

There's a translated edition of Thibaut's charters and letters by Theodore Evergates, who's done a great deal of research on the County of Champagne. Might be a useful tool for checking your work, or fleshing it out with context.