I think you'll be fine for your drive back. Many people (I believe most people) knowingly or unknowingly run aftermarket rims without hubcentric rings. Just make sure you torque your lugnuts on correctly and evenly. This'll sound weird but also try to make sure that the wheel is seated as centered as possible.

That being said, using the rings is the best thing to do. I haven't picked any up myself even though I've been meaning to for years now...but I drive on my steel wheels most of the time these days.

To measure them, you'll need to measure the inner diameter (this can probably be found on the internet since it'll be common for most civics and integras) and the outer diameter. The inner diameter will be the lip on the hub. The outer will be the diameter of the space behind your aftermarket wheels.

I believe Golden Apple distributes rings so they may be just one source if you can find a distributor. As CivicSiRacer has stated though, there may be differences in quality between different brands.

Once again, I think you'll be fine for your 800 mile drive. I've driven further without hubcentric wheels on. If you're leaving for school tomorrow, you've got other things to worry about and there's no way you'll get them by tonight. Just order them or buy them when you get there.