Drifter, the flexible additive Pontiac Dad referred to comes as a liquid you have to mix. If you are happy to spot paint the area, (instead of the whole bumper) you can try a PreVal sprayer. It is a little kit that sells for about $10 that lets you mix your own paint, reducer, flexible additive, or whatever in a 4 or 6 ounce bottle that comes with its own little separate aerosol propellant can that screws onto the top of the paint bottle. You can buy extra propellant cans and bottles too. The quality of the spray is not as good as a regular gun, that is, the propellant doesn't provide anywhere near the pressure you get from a regular spray gun, so you have to keep the paint on the thin side. Also, It is hard to get a good match when you are spot painting.
Option 2: remove the bumper, clean it very well to remove any silicone or wax BEFORE you sand it, sand it to 320 all over and take it to a body shop to have them shoot it. I did this with a hood once, and the guy only charged me $75 for a professional job, cause he didn't have to remove, prepare, or install.