This is being taken from existing US stocks.
What was done in the past is older versions (including those near or past their "use by" dates) were shipped from US stocks and the Ukraine aid money was used to replace them with newer ones.
The Pentagon was dinged for this by the GAO. That's partly how the Biden regime suddenly "found" additional funding, by revaluing what was shipped based on it being old vs. the cost of replacement with newer equipment.
If we have such a problem with the supply chain, then we can wait until sustained profits cause the contractors to acquire more plant and equipment or the US gov can pay for new plant or equipment. If shipping arms to Ukraine is such a strain, there's a much bigger problem than just supplying Ukraine with the piddling amount we are sending.