I recall a firm called Bullet 50 that was selling a line of black & dark blue jeans for LE, undercover cops, federal agents etc. I'm not sure of the URL. It was started by a group of retired-former south CA area LE officers.
5.11 is well known for the office casual/practical pants. Blackhawk sells them too now. Galls, QM Uniforms, Duluth Trading, & Streichers market pants/slacks for CC-plain clothes use(s).