I've been using the carbide countersink I got from Danny Proulx (sadly now deceased), however the business carries on. Under bits and blades here http://www.cabinetmaking.com/pages/hardware.htm. You could likely get it directly from Sharpco as well. They last a very long time unless you drop them on concrete floors, breaking the carbide. I've drilled thousands of holes with mine (second one) and it's still sharp. FWIW.
I have drilled thousands of counterseats with a HSS bit w/o dulling. Especially in MDF. When I am countersinking I do the countersink before the drilling the hole, As you (I assume) know where the hole is to be, it is no problem to set up ad drill the counterbore first, this eliminates the chatter created when you drill into a hole, making a smooth tapered divit. Then when you set up to drill the hole it is easy to find the center and follow through (point it down the funnel) Often it is assumed that the bit is dull because of the chatter bt a slight off center of the countersink to the hole will create vibrations and resulting lines or out of round in the countersink. I always Countersink first and drill second.
Already have a stanley/ryobi set, looking for a decent one for #8 screws, suggestions? (other than give up on mdf/melamine and use real wood)
I have a set of those that I got from Home Hardware years ago (Vermont American) and they work fine. I do not use them every day but I do use them in MDF fairly frequently with no particular dulling that I have noticed. I bought other countersinks from LeeValley that I use in my drill press only; http://www.leevalley.com/hardware/pa...3,41306,41328; they are very good tools IMHO and are similar to the deburring tools that I have for metal working.