Here are links below that sell sheet music/cd accompaniment sets:
http://www.musicminusone.com/
http://www.sheetmusicplus.com/
http://www.amazon.com/Celtic-Music-Flut ... 400&sr=8-1
http://www.amazon.com/Celtic-Music-Flut ... 400&sr=8-4
Band in a Box is software that lets you easily create play-along chord backing tracks from many included real instrument sound files: http://www.band-in-a-box.com/
I was the same as you when I first started playing the whistle. I know, I know, the whistle is easier than the flute.
Whenever people would watch me play, I would freeze up or make continuous mistakes. I made some guitar backing tracks to play along with, and that helped me a lot, and I bought some play-along cd's to practice with. Plus, I had my wife start watching me play at home. As I become more experienced with the whistle and the more I learned the songs, the less nervous I became and the more confidence I had. But I am still by no means at professional level. I still make mistakes. What you need to do is just practice, practice, practice until it becomes automatic. Until you don't even have to think about the fingering or embrochure. I do realize that it probably takes much longer to reach this level on the flute compared to the whistle (no embrochure skill on whistle), but I know if others have done it, so can you. Another thing I do is just focus on my music, block out the rest of the world while I am playing. Kind of like a playing meditation. Focus on the music and the instrument, not the people around you who are listening and watching. Another thing I would advise against (at least for me personally) is to not switch back and forth between different instruments while still in the beginning to intermediate stages of learning the flute. I did this for a while, constantly switching back and forth between the whistle and recorder. It seriously slowed down my whistle progression. So I started focusing on just the whistle, and my progression really pickup up again. So I have been just focusing on trying to reach higher levels on the whistle. But once I get the Tipple flute I recently ordered, I will only be focusing on learning it, and not the whistle. 
Something else I strongly recommend: slow down! Slow and correct playing is so much more important than fast and sloppy playing. I have seen way too many beginner-intermediate whistle players try to play ITM very fast, but sloppily. This only reinforces bad playing habits. I know most want to play fast ITM, but we have to learn how to walk before we can run. 
I don't play at sessions or concerts, but I do play Irish/Scottish airs and ballads on my low D whistle at a large crowded park where I live during the summer. When I first tried doing this, it was scary! Lots of people would stop and watch me play. I kept freezing up or hitting the wrong notes. Totally embarrassing! But now, it's not much of a problem. Stage fright is common. Just give it time. Play in front of a few family members or friends at home first, plus play along with some backing tracks or cd's. Given time and lots of practice, your confidence will build, and you will feel more relaxed. Then once your confidence builds enough, go to a park and start playing in front of strangers. It's actually a lot of fun. Live music seems to attract people like flies to honey.
