Unfortunately, it's not possible after that month's contest is over to go back and view the entries from previous contests by viewing the thumbs; that only shows you the current contest. We do have a 'Hall of Fame' thread that is supposed to have the winners of each month's competition linked, but we have been sorely failing in that for the last few months (years?). I hope to correct that in the near future.
Also unfortunately, the Advanced Search option seems to be on the fritz and I don't know how to fix it.
As of now, the best thing I can tell you is to find the voting thread for the relevant month's contest (i.e. this thread). That'll include a list of all the entrants and the titles of their threads. From there you should be able to hunt them down individually.