"on the lot" does not really make sense in this sentence. "of the lot" might be slightly more correct but the more correct ending would be " we could get the best Christmas tree that there is/ the best Christmas tree available/ the best Christmas tree money can buy

Thank you, DavidK. That was very helpful and easy to understand!

Christmas trees are often sold on Christmas tree lots. A “lot” is a plot of land designated for a business purpose such as parking, selling cars, making movies etc. [American usage] Christmas tree lots are temporary businesses that open several weeks before Christmas. Customers can drive to the lot, select the tree that they want to buy, and bring it home on their vehicle. Getting the best tree on the lot just means that they will buy the best tree that they can find for sale at the Christmas tree lot they will go to. “I don’t’ like any of the trucks for sale on the lot” (car) “I heard a rumor that ScarJo is on the lot” (movie) There is also an expression “of the lot” which means “of the group”.