Loads of phone networks sell their own WWAN cards, 3 are one the popular ones in the UK, see their website for details. The only problem could be that the device they provide will probably be locked to their network so you'd need to unlock it if you wanted to use it abroad with a local network there. They might help you out with this and tell you the unlocking code or you might have to find a way to get someone else to do it. I unlocked the built in WWAN card in my laptop using a program called dc-unlocker.
Or you can buy your own WWAN card independently, unfortunately I don't think OcUK sell them though and I can't link you up here. Google is your friend! I think the fastest ones available are HSDPA, and should be GPRS/EDGE/UMTS/GSM compatible as well. I think all the cards are multi band but if you're thinking of going to a specific country make sure that the card you buy can use their frequencies.
I've just been googling modems to help get you started and found the following models;
Novatel Wireless Merlin XU870 3G HSDPA 3.6/7.2 Mbps ExpressCard
Huawei E220 HSDPA USB Modem
I haven't read up on whether they are any good but it'll do to give you an idea of what i'm talking about!