Hi truckman
You might try Nate Merrill at North Sails Direct back east.
nate@direct.northsails.com
He was very responsive on the phone and with emails, unlike some of his cohorts back there. If you deal with North locally, they have to charge sales tax, which the outfit back east has a way to avoid. Shipping was something like $20.
We had him build a no frills 120 genoa with rope luff (North's better idea over foam) and the price was very competitive with the "off the shelf" internet sellers, and for a custom designed sail no less. It is on a Harken furler and we are very happy with it.
IMHO, and I know I'm about to get blasted, I am not a big fan of a 150 on the 310. I have no interest in racing or putting the lee rail under. Had a 155 on a Catalina 36 and even with a double reefed main, it was a handful over 15 kts. I don't like charging around with a partially furled headsail, trying to find the right place for the sheet leads, and I don't think it's good for the sail, but that's just me. I also feel that the single reef point on the C310's main doesn't reduce the heeling moment as much as I've experienced on other boats. With a single reef and a 150, I think you'll be partially furling the genoa in a breeze unless the attached picture is your idea of a good time.
I'm sure you'll get alternative opinions to justify your lust for a bigger is better, size matters, sail.
Best of luck.
Jack
Often wrong, but never in doubt