Can you buy foreign stamps at US post office?
No, the United States Postal Service does not sell foreign
postage. The reason is that postal policy specifically says that if
you are mailing something from the United States to a different
country, you must use US postage. Post offices will not process
mail with foreign postage on it unless it is coming into the USA
from a different country. So there is no reason for the USPS to
sell foreign postage for this reason.
Answer: The USPS actually has to accept foreign postage from any
country through which the mail piece is going to travel. This is
guaranteed by the Universal Postal Union (UPU) agreement. The UPU
is a UN agency that regulates the price of International postage.
If you use foreign postage stamps when mailing something to the
country that issued the mail, then your mail will be shipped there
and the foreign country pays the USPS.
If your local post office rejects the mail piece with foreign
postage, then you should let them know that such refusal is in
violation of the UPU agreement and possibly other law between the
Nations.
While I lived abroad, I have often seen British and German
postage meters/stamps on letters from the IRS.