I am a pretty good player, but can't seem to hit the higher notes of the upper altissimo register...don't know if it's my mouthpiece or something else, ie. ligature, reed, etc. I currently use a Yamaha 4C mouthpiece and a basic metal ligature with 2 metal screws, and LaVoz Medium Soft reeds.
Wondering if there are any good mouthpieces available for under $150 anywhere. Please help!!!

There are plenty of Vandoren mouthpieces under $150 that are good. The new M15 and M30 have good response and tone. The B45 Lyre is also pretty good. I would try a different kind of reed and see if that does anything. Try a Vandoren V12 or 56 Rue Lepic or i would also recommend Zonda's. Those are the reeds that I have found to be the best. Also, make sure the voicing in your mouth is right and your tongue is in the right position and your embouchoure is tight and all of that.

i would try a slightly stiffer reed, which might help with higher notes. when i used la voz reeds for a few months, the medium-hard's i used were about the same as my vandoren 3's.

mouthpiece-wise, my teacher recommended to me Roger Garrett's mouthpeices, which are pretty (suprisingly) cheap, but the one i ordered hasn't arrived yet so i can't tell you how it plays personally.