For some things (like the more popular ancient Roman or Greek works), it's not so much a problem, but for others, it seems I can only find translations. Specifically, I'm looking for the original text of The Travels of Marco Polo, in the original Old French. Where could I find something close to an authoritative text?
Some basic general tips which have worked well for me over the years:
Do your searching in that language. If you're looking for a book in French, you should use whatever search tools are popular in French speaking countries. If you're trying to buy a paper copy, for example, then you want to use French Amazon
If it's old, as in well out of print, check sites like archive.org or WDL (edit: also as termins-trantor suggested)
If you're searching for a specific text, be sure to check all of the names if it hsa more than one. In your case, it's also been published as Devisement du monde
Always check your main libraries. For well known older texts like this, major national libraries will almost always have copies. If you're lucky you can find it in State/Provincial libraries as well.
Sometimes you can find stuff on sites that share pirated texts. I'm not recommending that here, but a lot of older Chinese texts which I've needed but were otherwise out of print and impossible to find at bookstores will sometimes appear on such sites. Before they were shut down by the government, that is. So again, I'm not saying pirate stuff. I'm just saying some people pirate stuff.
Specifically for Travels of Marco Polo, is this 1350 digitized manuscript what you are looking for?
Also (and I really am not trying to be condescending) but often you can find links to digitized versions of old manuscripts linked on Wikipedia pages of the work you are looking for. The more famous ones at least.