When looking for boards where there are models made with lengths under 150cm, I'm having a hard time finding any in stock on various websites. Mostly, there are only versions available in 150+. Is this an end of season, exhausted supply thing or do companies tend to make a limited quantity of boards in the 145-149 range?
Combination of both, putting you at quite a disadvantage. You will obviously find the most stock of everything at the beginning of the model year (which for snowboards is late summer). At this point, everyone is just trying to sell whatever stock they have left before the 2010/2011 boards arrive.
As for manufacturing distribution, it would make sense that there is less made in that sub 150 cm range, particularly in men's boards. If you are a manufacturer, you want to make enough boards to fill your largest potential market, so you gear most of your manufacturing to the general average rider dimensions. For men's boards, that will generally make most boards 151-162 (not easy finding stuff bigger, but you can). For women's boards that's largely 136-149 (just from what I've seen; don't shop the women's section too often).
The two factors coupled together have you in a bind unfortunately. If you fit the weight and waist dimensions, the board is a good size, and you don't mind the graphics (or the stigma...
), there's nothing wrong with riding a big youth or women's board (if you're a guy).