You may also want to check out
http://www.lincolncentresource.com/index.html and see what they have there as well. This site has helped me a lot on what to look for as well. This will give you certain dates to look mainly for, but any coin may be an error. The other thing I did was went and bought a book about error coins which also let me know of other things to look for. You do not need to go and buy it but may want to go to a look at a local book store and check it out.
Odds of finding them are impossible to say. I have been searching for 6 months and have found about a dozen out of a couple hundred dollars worth of cents. The majority of errors I find are 'easier' ones to find and not worth a lot but fun to find and a new addition to my collection.