You've got really nice clubs -- why do want new ones?
Well back to your original question. I built the set of irons I am playing this year (see my sig, WITB). I'm really pleased with how they turned out but I built a 7 iron first and hit it for a while to make sure I liked the shaft/head combination. They cost me about $185 total to build, far less than the $1000 or so it was going to cost me for the clubs I wanted.
You won't find any "big name" OEM heads (TM, Titleist, Callaway, etc.) for sale new, they just don't do it. There are several club component manufacturers that do sell some very excellent heads and of course shafts, grips and other clubmaking supplies and tools too. The two biggest are probably Golfsmith and The Golfworks. You might browse their websites and see if any of their designs strike your fancy.
I got into clubmaking by starting with putters. Then some wedges and finally some full sets.
If you do decide to make your own clubs, try to go to one of the Golfsmith clubmaking seminars. They are really a good place to start and they'll cover the basics plus you'll get to ask an experienced clubmaker any questions you might have. Read up and study about it all you can. Clubmaking is not hard and doesn't even take that much equipment but if you don't understand the proper wat to do it you can mess it up royally. There are also several good clubmaker forums online too -- Golfsmith and GolfWRX are two that I get some good information from.
Here's a few pictures of the irons I built and am playing this year:
Good luck whatever you decide.
