Look on DaBay for a King 2340. They usually come in cases which means they were used by schools but are in OK condition because they were transported in cases. They can be had reasonably- $500-$750 in fairly good shape. And it's a good tuba. Three valves but a fourth valve is gonna cost ya and it has good intonation and can be played well. I use a Helleberg with mine. Any of the 'standard' tuba mouthpieces will work (7, 18, 24AW, Helleberg). If you wanna play the mouthpiece dance, I'm sure people will have suggestions for you. Usually comes with recording bell. Upright bells are scarce and will cost almost as much as the whole rest of the horn with a recording bell.
Jeff "In the basketball pep band this winter" Benedict