Requests for loans or photocopies from libraries outside the UK can be made via the BLDSC or direct.
BLDSC's prices for International document supply are available here: Price information
Information on finding locations and making applications, (including payment methods for international requests) and also on supplying material to an overseas library can be found here: CONARLS International Requesting
The International Federation of Library Associations and Institutions (IFLA) runs the IFLA voucher scheme (payment for international transactions).
There are a number of useful union catalogues to help locate material held outside the UK:
Libweb index - Library Servers via WWW
Libweb currently lists over 8000 pages from libraries in 146 countries