I purchased a prepaid card from Telestial for China and I'm sure that there is no problem if you begin using them within a few months, however, the only reason I purchased that one was that I wanted to have the number before I left. Typically, you can save at least 50% by purchasing the card once you arrive in the country. For example, in Singapore, if you go to any of the Moneychanger booths in the airport the SIM card is free if you purchase a balance on the card. In the UK, you can purchase an Orange SIM for about 12-15 pounds.