Welcome! I grew up in Omaha, first learned to sail on some of the small lakes around there, and also did a bit of sailing on Lake McConaughy. Nebraska doesn't have a lot of lakes, but there are plenty of opportunities to hone your sailing skills around the state.
As for a mast and boom, there is a Yahoo group for Holder 12 owners; should be able to find some help there. Holder12 : Holder12