I'm the guilty one here, I've bought every single M203 barrel, 9" or 12", on the market that I could locate for the past year or so, excluding the EE here. I've bought somewhere in the neighborhood of 200 barrels in the past few years, and started buying all of them I came across for the last year or so. I haven't found a better source than the secondary market, sadly. I could probably part with one, but you'd have a better time ordering direct from LMT once your F1 is in, in terms of price.
I use them for other projects, not even on a M203, and it hurts every time I have to cut one up, but I can't find any other source for 40mm barrel blanks. I end up using M203 barrels, M79 barrels, and even Mark 19 barrels. Hopefully I can work a deal and find some 40mm barrel blanks with the proper rifling, but until then I just buy all the M203 barrels I can find for sale, so... sorry about that.