I'd check the EO tech or Trijicon www.trijicon.com
They are popular with SWAT, spec ops, armed citizens.
A front or fore-grip with a green laser aimer & white light is slick too; www.crimsontrace.com www.sigsauer.com
There are many add-ons & after market sights or parts for the M4s and AR rifles.
has a product catalog of just AR rifle stuff.
Here are a few other top brands; www.policehq.com www.uscav.com www.actiongear.com www.natchezss.com www.midwayusa.com www.grafs.com www.qmuniforms.com
Mag-Pul is highly rated for AR type magazines. HK's newer style AR mag is good but they are rare & the price may be sky-high now(2013).