Reddit mentions of ONYX Adult Oversize/Super Large Type 2 USCG Approved Life Jacket

Depends. I tried using the shuttle once and they refused to take me since I didn't have a "dual-chambered" tube, which is bullshit. It was just their way of forcing you to rent one of their tubes if you want to ride with them. Taking 2 cars is definitely the way to go, leaving one at the exit point (Parking costs $3). If you're going to go more than once over the summer, just invest in the tube a life jacket. These are the tubes everyone buys, and buying it costs the same as renting it once. It won't help you for tomorrow, but life jackets are currently $12 on Amazon. So once you make the initial investment, it's free to go as much as you want after that.

I use a different route than what was suggested above: Start at Cochran island, 5450 Interstate N Pkwy Sandy Springs, GA 30328 and end at Chattahoochee River National Recreation Area: Paces Mill
3444 Cobb Pkwy Atlanta, GA 30339. I've done a lot of tubing and this is the best route, and it's very popular. It takes 3 hours. I've never heard of being able to rent only the life jackets, but it may be possible.

I go pretty often, so let me know if you have any other questions. Oh, and here's where you check if it's safe to go, but this is only an issue if it's been raining during the week.

EDIT: I just saw an even better deal for life jackets