Try www.galls.com. They sell brand new, NIJ 0101.04 rated IIA vests for $239. Second Chance vests are probably the most comfortable (depending on model purchased) but they also tend to be the most expensive as in the Ultima 9th Gen. If you're not going to be wearing the vest day in and day out, you might not need to spend the money on the most comfortable vest. Many of the big name companies sell good quality vests for under $600. Soft body armor is like motorcycle helmets, they all protect the same at a given rated level, but the increased cost usually translates into improved comfort/wear ability (the theory that a thin IIA vest on the body protects better than a stiffer IIIA vest in the locker or trunk).
In my opinion, the best light/comfortable vests under $600 include: the Second Chance Superfeatherlite, Safariland Zero G, PACA KGS, etc. Just make sure to get one that fits.
Hope that helps some.