You don't need cell service.
You probably want a GPS source of some kind. Your options for that are:
1) Buy an iPad with 3G/4G cellular capability, which will have a built-in GPS. $130, gives you the option to add cell service if you can't find WiFi and need a data update somewhere. Also gives you the option to check weather from anywhere, which is very nice.
2) Buy an external GPS. ~$99 plus shipping, and another battery to keep charged.
3) Buy a Stratus. ~$800 but gives you traffic and weather in flight with no subscription fees. Probably overkill in your case, UNLESS you're flying an airplane equipped with ADS-B Out in which case it might be worthwhile for the traffic.
Simply not true. Want proof? Do exactly as comanchepilot says.
I used an iPad with its internal GPS in flight for years (usually use the Stratus GPS now). I had exactly ONE issue with it, solved with a reboot.
aGPS does not need input from the cell system, the cell system simply allows you to get a faster lock when the device doesn't know where it is. If you turn it on prior to takeoff you'll be fine. If you turn it off, fly several hundred miles, and turn it back on above 10,000 feet you may not get a fast lock. However, this is far from how most people use them. Mine's on from startup to shutdown.
Older devices could lose their lock at 250+ knots up in the flight levels, but that doesn't seem to be a problem any more, and I doubt that applies to anyone going 200nm VFR anyway.
Also somewhat misleading - Despite what the dialog boxes may say, the only thing that WiFi or cellular signal gets you is a *faster* initial location. Once they've got a GPS lock, it's as accurate without any other signals as it is with them.