can i buy internet for wifi only ipad

You have a couple of choices

Only use public wifi or existing wifi hotspots

Subscribe to the internet from a local provider

Purchase a device like the Mifi, which uses cellular to make a local wifi hotspot you connect your ipad to

What is best for you depends on where you are and what your budget is

Do you not have internet at home?

How are you connecting to this website right now? Why can't the iPad use the same internet you are using now?

If you want internet when away from your home Wifi however, in addition to the great options given by Skydiver119 if you have a phone that supports it, you can use Personal Hotspot to share the internet from it to the iPad.