There are places that mix paint, and will put it into an spray can for you. There is a Napa in MN that does this, so check around. I do not see how adding a flattening agent to the paint would be that big of a deal. Not too sure about etching primer, but I am sure that anything can be done, for the right money!
Go to Home Depot or Lowes, even Walmart, and look through the spray paint. There's a number of flat, satin, and various texture finishes available.... it is crazy. Check the manufacturer's webpages for the whole spectrum. You can buy the self etching primer in cans, I have a couple out in the garage.... don't know the name, let me know if you need it.
Duplicolor and Plasticote both make a beliner in a spray can these days... used that on the runner. Decent stuff, but there is better.
Hammerite! is thicker than regular paint, not glossy but not flat either. I was trying to find the url for them but I wasn't able, if you do a search on google you can find a few places than sell it though.
I personally haven't done it myself but there are a few cruisers I've seen with it and it looks good, I am using durabak on mine sometime next week.
I've found self etching primer at my local auto supply store (Smyth Automotive) and they can also flatten any color add flatener and put it in a spray can for a decent price. Guy i know rattled his whole jeep a flat navy blue this way and it looks good for the $$$.
Cool, thanks guys. I checked with my local auto parts store and they do have a metal etching primer in both spray and quart contaviners, but no flat colors other than the basic krylon stuff. I will try the local paint store next to see if they can put it in cans. I had previously checked for it in cans there, but they did not have anything.