At first I rode my kart at a large secluded parking lot behind a vacant industrial building that I found near my house, but before long I got bored just riding around by myself. So I posted some craigslist ads seeking other guys with karts, and asking if they had any good places to ride.
I got several responses to my ads and I ended up having them join me at the parking lot I'd found, and also joining them at the places they'd already found to ride/race their karts. We usually rode on the weekends and we've never had any trouble with the cops so far.
If you wanna try posting a craigslist ad like I did, just keep in mind they dont have a dedicated Go Kart section, so all the go kart ads end up getting posted in several different categories. I'd suggest you post your ad in the Recreational Vehicle section, and then at the very bottom of your ad, include all the different spellings for Go Karts that are commonly used on craigslist like this:--> (go kart - gokart - go cart - gocart)
This way more guys using the craigslist search window will be able to find your ad.