If you have not yet purchased the iPad, of course you can purchase one with built-in cellular data. These iPads (Wi-Fi + Cellular) are ~$130 more than the same spec Wi-Fi only models. On top of that, you need to setup a specific data plan for the device and pay for monthly data.
I have Wi-Fi only iPad models but use my iPhone's "Personal Hotspot" feature. This allows your iPad to access the phone's data from your existing cellular plan (your iPhone creates it's own Wi-Fi network which you connect the iPad to over Wi-Fi) without having to setup the iPad's own specific data plan. It's saved me the additional $130 charged for the Wi-Fi & cellular iPad models as well as the additional iPad data plan charge per month.
It may only work on certain wireless carriers and those carriers may charge extra for it. My iPhone is on Verizon's "Share Everything" plan which allows Personal Hotspot for no additional charge. I can't comment on other carriers though.
Bottom line is you do not need to buy a Wi-Fi + Cellular iPad is access the internet while away from Wi-Fi.... you can possibly do it through your current cell provider using your phone or you can buy a mobile device (mobile hotspot) that will let you do the same...although having to carry around only the iPad (Wi-Fi + Cellular) would be nice.
