I run an online texting site and really, it's a pain in the ass for international using just a script. You have to use the gateway, basically just an e-mail address (for AT&T for example: ##########@txt.att.net), to send the messages. A lot of international providers only allow their users to receive messages through this gateway if they've opted-in to use it. This entails the recipient doing one of various things, such as sending a certain text to a certain number ("ON" to 212, for some UK), going to a web site (for Bulgaria), or dialing a certain number (UK and Spain).
The only way around this is by using a GSM modem, which is basically a "modem" that acts like a cell phone in that one if its primary functions is to sit there and send out texts all day. However, these require wireless provider subscriptions (on your end), though it is a much better method because it sends the messages as if they're coming from a real cell phone, so no weird activations required. This does require extensive knowledge of programming for initial configuration, so if you can't set it up, you would need to hire someone.
For a list of SMS gateways and any activation methods they may require:
http://www.mutube.com/downloads/mobile-networks.ini