You are right in my wheel-house after 16+ years of dress-suit-based EP work... Best option, as jcustisredux states, is custom tailor from the start.
You can do Mens Whorehouse or Jose`Banks, but I've found those suits tend to: a) wear out as fast as anything due to your gear and likely activity level, b) end up costing you just as much as custom when you factor in the alterations you need, and c) due to the likely alterations you very possibly end up looking like a soup sandwich in your big brother's suit. If you just need 1 or 2 suits for the urgent and/or occasional gig and you're an easy/off-the-rack fit, they are certainly an adequate solution, but if you're going to be dressed like this 40+ hrs/week (and particularly if your clientele is high profile and you're going to find yourself inadvertently photographed....), you'll be thankful you did it right by going custom.
There are some custom tailors that do alot of work for EP, cops, DSS, and USSS types. One that I've used with good results is Hong Kong based, but does a traveling roadshow around the US 2-3x per year, doing sizing and fitting by appointment, then ships you your items a few weeks later. He's got a full palate of colors, fabrics, price points, etc. and does shirts also (Note I HATE the fit of off-the rack shirts, way too baggy...) Its not uncommon to walk into an appointment with him and see guys doing a fitting in full belt/kit and low-viz armor, and they'll be simulating a shooting stance/sitting/squatting (crotch tightness is problem with the trend toward tighter European-cut suits...), and then getting into the finer points of size, quantity, and placement of pockets and belt loops. You'll want to do same if you want anything to fit right while actually working. I know he's coming for another round in July-Aug. PM me for details.
Lastly - There's also the http://www.coolactionsuitDOTcom supposedly designed with input from USSS/DSS types, but I dont know anyone personally who has one who can offer review. I'm wary of buying a suit off the internet without a proper tailor, but I'm hard to fit anyway..... YMMV.