Buying prepaid cellphone to use overseas?

This idea won't work.

Boost phones use the CDMA signal format which is not used much overseas, where GSM is more common.

On top of that, Boost phones (and pretty much all prepaid phones) are locked to only work with their original provider and can't be unlocked. They have to be made this way so that the provider won't lose money if someone buys their phone but doesn't use it on their service.

The type of phone you need is actually an unlocked GSM phone. With that kind of phone, once you get to your destination, you can buy a local SIM card and it'll work fine.

You can buy unlocked GSM phones from Amazon.com, eBay, etc.

You can also start with a locked GSM phone (the main GSM carriers in the USA are AT&T and T-Mobile) and then have AT&T or T-Mobile unlock it (they usually will do this for phones that aren't brand new).