There have been efforts to put together a listing on the internet of IRCS required by country, but to date, a reliable comprehensive list has not been developed. If in doubt, ask the DX station, and follow the instructions given. If you send a greenstamp or other currency, realize that there is normally an exchange cost to convert the money into local currency. Again, if in doubt, ask the DX station, or ask a friend who has received a card from the station to find out what worked. If you include actual stamps with your SASE for return postage make sure you send sufficient face value in stamps to ensure return.
Several vendors provide the service of selling mint foreign stamps to hams for SASEs.
However, most managers would prefer you NOT affix the stamps to your SASE, but rather include in your envelope. That way, if you send too much, the extra can be used by the manager. For an IRC to be valid, it MUST NOT be stamped in the right hand box. This implies that you expect the DX station or manager to pay for the envelope to return your card. If you use this method, you risk having your card returned as a post card, with all sorts of postal system markings on the card.