Transposing Practice?

[Jimmy Djents]

Hey, I'm no newbie at guitar but I have been starting to learn music theory over the last few months and one of my favorite things to do is transposing classical pieces of music used in examples in my theory class and to transpose it from sheet music and play it on my guitar. I would really like to practice doing this with intricate pieces of classical music until I can transpose on the fly.
If someone could give me a suggestion or even upload the sheet music so I could practice I would greatly appreciate it. I'm sorry if this is the wrong forum but I believe this is theory related, correct?

Also, I would like it if anyone here could tell me a good site to practice my ear training to, and how to practice changing keys besides just moving scale shapes up and down the fretboard. For example, when I play with other people, I can play about 75% of the time and it will sound good, I'll try and find the progression playing within those keys, but it's really hard to keep up while counting if I've never heard the music before. Is this something that I just have to practice or is there a better way to do this? Thanks in advance!

[SchecterWhore]

Welcome aboard this fantastic funky vessel we call the Music Theory Express. Awwwwwww yeah. You just sit back and prepare to have your mind funkified.

Ricci Adams' Musictheory.net
Ear Trainer
International Music Score Library Project (IMSLP) Portal

I have some sources for newer scores (read: not public domain) as well, so PM me if you're needing a George Crumb score or something.

Regarding transposition practice, I would get used to plying all of the intervals at every place on the neck. Choose an interval (let's do a minor third), then play it starting from Eb, so you're plaing Eb-Gb. Then, move it up a perfect fourth. Now you're playing Ab-Cb. Move it down a major seventh, then up a major second, then down a major third, then up a minor third. At this point, you should be at Bb-Db. Just get used to moving intervals around, and you should be good to go.

[Solodini]

Have a wee look at the free sample chapters of my book for ways to get better at changing key, without just shifting a shape. Link in my sig.

The gist is to learn the accidentals of each key/how to work this out so you then know what notes will be natural; learn to find each note on the fingerboard (I rely on intervals from the open string as my method, that way I'm able to quickly adjust to playing in different tunings); combine the two so you're quick at finding notes which fit the key at any location on the fingerboard.