Do I read it correctly that you already have a contract SIM with internet access on it? If so, you can use it in a dongle - if you can get one: I've not seen many SIM-free dongles for sale.
If it's a "pay as you go" SIM, there's not much mileage in trying to get a dongle for it anyway.
For reasons I've not worked out, a USB or PCMCIA dongle for mobile internet is expensive. My mobile phone contract includes more GPRS/HSDPA data than I ever need, and it works with the laptop on my desk and the phone in my pocket. That's the phone plus £7.50 a month for the data.
If you have a spare cellphone with GPRS/HSDPA capability, a USB lead for that (or Bluetooth) may be a workable solution without paying out a load.