paintscratch.com, dupli-color perfect match etc. usually these come in aerosol, or touch up pens, but also bulk for use in a paint gun. if you can get it that way through the companies, or locally as long as its the right paint code yous should be good. I've gotten decent results with aersosol, just have to take your time and sand and wet sand between layers.
since the trunk is a different angle than the rear fenders and bumper it doesnt need to be a 110% color match. if it were a front door vs rear door where they could be compared next to each other you would have to blend or paint both panels. but you wont notice it really on the trunk.