OP: Hey can you recommend a way to paint my toolbox?
GJ: Pinstripes
OP: Nope, cant.
GJ: Then go with PPG
OP: Nope, can't.
GJ: Alright, tractor enamel then.
OP: Nope, can't.
GJ: ...uh, then powdercoat?
OP: Too expensive.
GJ: Sigh. You can do it at home.
OP: Nope, can't.
GJ: Run by O'reilly then, they've got what you need.
OP: Nope, the most immediately close Oreilly near me only has Duplicolor.
OP, what exactly is it you want? If you want to paint this toolbox with something better than rustoleum, have you considered you're going to have to expend a few more miles/hours/dollars than rustoleum to get that accomplished? If you're not willing to drive farther than the closest autoparts store, use any appliances or tools other than what you have at your immediate disposal, do the red tape your state requires to use certain chemicals, or spend more money than what rattlecans at HD cost, you're not going to get anything more than just a spraypaint job.
Aside from that, there's been like 8 awesome suggestions in this thread that you've summarily shot down. If you're not going to make a little effort for anything more than spraypaint, why waste the board's time trying to help you?
Just my $0.02.
To contribute, your local powdercoater can do the whole box for something like six bucks a square foot. That's what I would do. It's cheap, it lasts forever, you can choose your color, and what extra you spend in dollars you will save in time not having to do any of it yourself.
Alternatively, the way you're approaching this topic, I think you should spray it with valspar and put 3 or 4 coats of Krylon Clear on it to protect it. That's your cheapest/simplest/most immediate/least effort in the world involved solution, and just about the only solution that hasn't been suggested yet.