This is what I've been using for many moons for a side boom...I like the adjustment that you can do quickly to get the mic closer if you have guitars that are different volume.
http://www.guitarcenter.com/On-Stage...nt-Boom-Arm.gc
Not sure what your mic budget is, but my current go to for miking acoustics is the Blue Hummingbird. If you are patient you can get one used for 150 ish - maybe a little less.
Other really good ones are Audix ADX-51, AKG C451 (more $$), AT4041 and in a large diaphragm, the AKG C214.