Ok so my girlfriends dad gave me a 1984 Oldsmobile Royale Brougham and the only thing wrong with it is the transmission because as soon as the car hits 30mph the transmission gives out. I‘m not a really big car guy so idk what to do. Where could I find a new transmission and is there a certain one I should look for?
You don't have to look for a transmission. Go to a transmission shop and have yours rebuilt. They have rebuild kits.
Most likely its a THM200 4R. Slight chance its a THM350c but that is very unlikely. The 200 4R was in production till 1990 but after 85 was pretty rare so these days there are not tons of them laying around. 80's station wagon or G body would be the most common places to find one. The G body was the Regal, MonteCarlo, Cutlass, and Grand Prix. The trucks used a completely different model trans so you can just swap a later truck trans in there. It was a fully mechanical transmission except for the torque converter that was run by the engine's computer. They were not known to be the strongest transmissions out there. Guessing that one just has a combination of age and mileage and needs a rebuild. Most transmission shops should be able to do the job. Parts are still available. Rebuilt they are stronger than the were when new.
Always check the vacuum line that comes from the MAP sensor to the carburetor for broken ,disconnected ,or cracked as the Map sensor controls the torque converter that propels you forward.