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This site has all of the information you need to file a case in Small Claims Court.http://www.courts.state.nh.us/district/claims/index.htm
The association will need a copy of the condominium documents, showing that the person agreed to pay a certain amount each month. If you had good association records, showing that everyone else had paid, that would be better. However, since he's the keeper of the records, the judge can actually consider the lack of decent record-keeping (I know, if I were Treasurer, I would make really sure there was a record of my own payments, if nothing else). You can also get copies of blank subpoenas
from the clerk of the court and serve them on his bank - you are correct that, even if the bank has merged or been taken over, they will still have old bank statements for at least the last three years. If he really wanted to, he could get the records. If you know the dollar amount that he should be paying, you can also ask the bank to send copies of all checks for that amount, or all checks made payable to the HOA.
If his defense is, "Wait a minute - I paid!" and he has no evidence showing payment, that's not going to work. The judge will also know that he could have gotten copies of the evidence from the bank, if he really wanted to, and will consider that he doesn't have anything.