Obviously an Airport Extreme or Express would be ideal, but I think you should try to create a wifi network from your iMac first. It'd be free, and if it works, why not?
On your iMac, click on the Wireless (Airport) Icon in your menu bar. It looks like a rounded triangle.
Click "Create Network."
Give it a name.
Check the box next to "Require Password" and enter a good password.
Then on your iPad, turn on Wifi and you should be able to find the network's name that you gave it, and surf away.
If this doesn't work well, then put some cash into an Airport Extreme, Airport Express, or a Time Capsule.
