Where can I find a lawyer for a contractor dispute?

The extra payment demand is only one part of the dispute, because the unfinished work may already be costing the business money. There may also be an arbitration clause or notice requirements in the contract that affect what you can do next. You can contact a commercial litigation attorney here: https://arkadybukhlawfirm.com/practice-areas/litigation/commercial-litigation/ . Bring the signed agreement, invoices, payment confirmations, and every written change request to the consultation. A clear timeline of the missed deadlines and unfinished work will help the attorney assess the case.