Hmm... Did you teach yourself?? If so, maybe you should consult your local music center(Music and Arts if you have one near) to get a teacher. (Yes, I know teachers cost money..But in the end, it'll be all worth it! )
I found some Tuba Mouthpieces...(
Tuab Mouthpieces)And about your range... Sorry, I can't answer that because I'm just a trombone player...But still, I hope I helped... And 5 years? Do you think its not good? My band director said that we think we play loud, but we plays so soft actually. He told me I played very softly, but I think its loud, It must sound like that because the instruments near us, but to a audience, it will sound so soft. So....To hit higher notes...You should take your mouthpiece, and buzz your lips, fast-like, and remember(Your gonna hate me for saying this...) Tuba is a BIG instrument, you need to use LOTS of air...Same with me and my trombone. I always have to be reminded that its a big instrument, unlike the flute...
Good luck and I hope I helped,
HM