Have nations buy units at the end of their turn.
Ships are still repaired at the beginning of the turn, though.
US Supply to UK and France: Starting turn 1, US has option to build supply tokens at a cost of 3 IPCs each. They are built like any other purchase. Then on the following turn, they can be transported to any friendly controlled territory in Britain or France, as a method to provide 3 IPCs per token to the receiving country. These tokens are transported like any military unit. They can also be transported by UK or France transports. The delivered supply tokens are included in the collect income phase.