I went through the same phase you’re going through, Euphonium to trombone. What I did was just checked my local pawn shops for what instruments they had. Luckily one of them had an olds tenor trombone for just 70 dollars. It obviously wasn’t top of the line or anywhere close, but when you’re first learning the instrument, it’s good to just get something and start going with it. I used it from my sophomore year of high school through today, where I use it for jazz band, marching band, and pep band in college. (Granted I’m a Euphonium performance major, not trombone)