You have several choices:
- If you have the iPhone, all the carriers allow you to set it up as a "personal hotspot", but it will cost you extra.
- I'm sure you can do the same with other brands of phones.
- You can buy portable WiFi's (called MiFi) from the major carriers (in the USA and Europe, I have no clue about other areas) that run off of cellular networks. The data is about the same price as the cost of data from the plan on the cellular version of the iPad.
- If you are in the USA, there a few carriers that offer something called Wimax, that allow for a portable WiFi that is quite a bit cheaper.
Mostly, you need to utilize the cellular carrier, and it's not going to be cheap. Expect to pay around $10-20 for 1-2 GB of data, more or less, depending on the carrier. If you're surfing the internet, that's not bad. If you're streaming media, it's too expensive.