Even though this question was asked a long time ago, it is still a relevant question. In the Boise area, there are very few independently buildable lots but they are out there. Most major builders develop out their own subdivisions or partner up to control an entire subdivision.
Occasionally, there are odd lots available.
In our area, most builders work through the real estate community and will not sell a home direct to the consumer since they want the Realtors to help through the process.
Most of the online public portals for real estate are based on houses, not lots. Therefore, team up with Realtor that has closed homes with a large number of builders in the valley so you can gain from their experience.
Hope this helps!