I found a brown ladies' button up blouse and khaki pants at Goodwill. The pants are a bit too snug, but the style is just perfect, and we all love Capt. TightPants. (It's entirely possible that they'll fit better once I reach my goal of running 70 miles this month.) I ordered some generic brown boots from Groupon; they're not perfect, but they fit my budget. If I can find/afford better ones, I might get them.
Tan suspenders were kind of hard to find until I stumbled upon the magic Ebay search term: "beige." I found a pair I liked at the right price point and just got them in the mail. They're button suspenders instead of clip-ons, so I'll have to get buttons to sew onto my pants.
Toy pistols are a bit difficult to find, too. Stores only sell neon-colored guns these days, and the ones on Ebay were $20+. Halloween City had me covered for only $2.99, and I'm pretty sure we have an acrylic color in the household craft room called "Gunmetal."
I already own a brown belt and had resigned myself to making a holster and Browncoat from scratch, but I took one more look at Goodwill and chose a cheap brown purse to dismantle and repurpose and a "beige" raincoat with very nearly the right cut and fit. I'm going to (try to) dye it and make minor alterations: take in the wrists, add cuffs, make side slits from hem to bum, take it in at the waist, and remove the cincher at back.
I will post updates once I complete the alterations. So exciting!