Anywhere they sell gift cards. Also you can use reload.com
If a store sells any gift cards AND they have that little terminal pinpad thingy then they should be able to sell a voucher that they print out of that pinpad.
If a store sells vouchers through their own system, then from a couple I've checked...they don't sell PM. But please do report any successes.
@Shakty Do be careful of where you get the vouchers. As someone mentioned in this community that the PIN is not working.