I have difficulty finding "ANGOSTURA" bitters, is there any substitute?
You could try Peychaud's Bitters (difficult to find), Fernet-Branca, orange bitters or other types of bitters.  Worcestershire sauce may also be used as a substitute but works well in savory dishes. I wouldn't recommend it for a Manhattan. 

Or, if you're very ambitious you can try to make your own bitters, although the ingredient list is somewhat intimidating! Good luck!

http://spiritsandcocktails.wordpress.com/2008/04/22/how-to-make-bitters/