I currently play a tenor yamaha allegro f attachment trombone. Now this is the horn ive played so far besides some rentals but they were pretty much crap. I really like tone of this horn. Now ive heard and read that yamaha's aren't really that well for tone and quality. Im switching over to an alto trombone cause i play in two different jazz bands i need those high notes. can i get some help finding a nice alto trombone or at least a good brand of one so i dont make any mistakes...

I hate to break this to you, but you're not gonna have much more range on your alto than you do on your tenor. Either you can buzz the pitches or you can't. There is more resistance and that may buy you a note or two, but don't think going to an alto is a miracle cure for your range issues.
If I recall, the Yamaha Allegro is a large bore instrument, and for your jazz band and ska music, a small more tenor may be more in order. But keep in mind, people play things on alto for the difference in timbre, and most often because of its ability to not over power singers in the choral pieces.
Alto trombone is not easy. There are a lot of tuning issues to work out, all new slide positions to learn. It will take you quite a while before you're ready to sit in your jazz band and read your charts, much less improvise a solo.
But , if with that said, you're still die hard about owning one, Courtois makes nice altos, so do many of the other makers. But in your case, i'd say get a dirt cheap Selman alto, get a slide job and a new lead pipe, and you should be good to go for under 300 bucks.

Thanks for that because that may have been a mistake...I really do love to play trombone and I wanted to try something new...I am willing to learn the new postions and notes and all but were do i get the lead pipes and a slide job done?

Also, i've been researching this cheap Selman alto. A lot of people have changed the leadpipes and got a new mouthpiece and said it was 200% better. They also say it costs about 350$ with the new leadpipes and mouthpiece..Would you recomend that idea. Also, if the only purpose of the alto trombone is the timbre, and its so i dont cover up the vocalist, should i just switch to a small bore tenor or what? now im debating between either a decent alto or a small bore tenor.

You'll get far more use out of a good small tenor. Plus, an alto just won't have the power you need for a big band setting more often than not. I know Doug Elliott is making use of alto in big band settings, but let's face it, most of us can't play like him. If you're considering Bb's to be "extreme" range, you're just not ready for an alto. Your money would be better spent on a small tenor, maybe a good used King 2B, and lessons with a good teacher who can help you with your range issues.

Do you have any cheaper recomendations. I am looking for one under about 1,000 and i can't seem to find a used 2B