older models

Apple has a refurbished shop that has some older models. Best Buy may also sell some older devices.

Certified Refurbished Products - Apple

Just be sure that the activation lock is turned off

Find My iPhone Activation Lock - Apple Support

So you may wish to avoid non-regulated resale sites, or if you choose to look for places like Amazon, Craig’s List, etc, educate yourself about the activation lock and be sure you get a functional device, not something that is an expensive brick.

You could go to ebay, Craigslist, or ask around. I'd go to the Apple Store and buy the $339 iPad.