How can I purchase more space on my iPad in order to download the latest upgrade?

You cannot purchase any more space. Try the update connected to iTunes. That does not require the space needed to do a wifi update. The other option is to delete something off your iPad, do the update, then readd it.

Connect to your computer, select your iPad under devices on the left side, it should open the main summary page, click the update button in the version section.

Settings>General>Usage You will see how much space is available. You will also be able to see which things take up the most room and delete things. Remember, you need 2.5 gigs free on the iPad to do the upgrade.