I'm just the guy to ask, as I just recently did this myself.
There are 3 AND 4-disc DVD cases available that match a standard DVD case's dimensions just fine; I just ordered a few of each to box up my own PS1 multi-disc games, specifically my copies of FFVII (3 discs), FFIX (4 discs), Parasite Eve (3 discs), and Super Adventure Rockman (3 discs). I cased them all up using unmodified TCP covers (save for Super Adventure, which I still have to make myself).
Most 3-disc cases will have a standard DVD spot on the inside back, in the normal place you'd expect to see one, and the other two discs being stored on a hinged insert. These kind usually also have the tabs on the inside front cover to hold manuals and the like. The 4-disc types that I ended up buying have places for disc on the front inside, the rear inside, and a hinged insert that holds the other two. These kind do NOT have tabs for manuals since there's a disc in the way on the inner front. However, there are also versions which feature overlapping disc slots which hold two discs on the inner front and two discs on the inner back, and do not feature a hinged insert. I have not yet seen a 4-disc DVD case that has tabs for manuals.
I bought mine at
Square Deal Online. I went with them because they're geographically close to where I live, plus you can only order a small quantity of them depending on what you need (I bought 5 each of 3 and 4-disc cases). They're pretty cheap, and the quality is quite good, at least from what I received. Below are the links to the exact products I bought:
3-Disc DVD Cases4-Disc DVD CasesThat site also has replacement Blu-Ray, Wii (white DVD case), PS3 (clear Blu-Ray case), XB360, GameCube (2-disc variant) and DS cases for sale.
...Wow, I just realized that almost came off as an advertisement. I am NOT affiliated with those guys, but I had a good experience with them, so I wanted to let you know. Cheers.
