You are going to be so happy that you are getting a larger tank. Something to think about - a 75 or even a 90 gallon tank has the same foot print as a 55 gal. All are 4 feet long - the 75 and 90 are simply taller and a bit thicker. I just bought a used 90 and was originally going for a 55. I'm SO happy that I did. And I can see how attached you are to your Pleco so the bigger tank would be great for him.
You might get lucky and find something on Craig's list.

Quick IMPORTANT note: When you do get your new tank - no matter what size - you cannot simply move your current fish into the tank. You have to either cycle the filters first (which can take several weeks) or MOVE your current filters with the filter media on to the new tank. If you don't, you risk killing all of your fish.

There is GREAT info on this site for cycling a tank.

Good luck and it's great that you want to do the best thing for you fish!
and happy birthday soon