Here is another idea. Google has a lot of patents. http://www.google.com/patents (Moderators, I know that you don’t post links, but if you can’t post this, then just post Google Patents). Obviously, I would not rely on this as a search, but you can search google for keywords that have to do with your invention to get a sense of inventions already out there. Again, I would not rely on this as your only search. There are many more things that you have to look for.
You can also find a list of agents and attorneys by searching: https://oedci.uspto.gov/OEDCI/query.jsp (or if Moderators do not let that link go through type patent agents into google and click on the first link (should be the OED of the USPTO)). The higher registration numbers (60,000s mean newer attorneys are agents). Some of the lower numbers may have retired. Maybe finding a 40 or 50,000 registeration number would be good. You can also search by geographical area.