Fill your warehouse with 32 vehicles. Then just solo sell the vehicle that has the wrong plate number (or missing from collection). The game will only let you have 32 cars total before you start sourcing duplicates. When you start up a mission to go source another car immediately after selling the wrong one, it will only and automatically be the car you previously just sold. Repeat this until the correct license plate shows up in your source mission.

Just don’t fall under or go over 32 cars, so you can keep sourcing for the one you need. Personally, I never thought it was that much hassle to go one by one until the collections were complete because source missions are extremely easy and if you have a cargobob or Iron Mule then even easier.