If you're able to get a small ceramic heater, you'll find that there are many unexpected places to plug it in. E.g. near gas station car wash exits, some parking lot lamp posts, etc. Check with the owner for permission, there are some that will grant it, especially if you're able to offer some lot cleanup or other bartered assistance for them.

Don't use a propane heater, please. It's dangerous to you and potentially to others nearby.

Source: Been there.