You all know the shirt I mean. No BBC Sherlock cosplay is complete without at least one purple shirt of sex. Buttons that don't *quite* stay shut, a flattering shade and cut, combined with rolled up sleeves to create the illusion of this being a utilitarian shirt. We all know what you're trying to do Sherlock. You look good. Here are a few that might work if you're not resourceful enough to make a tailored shirt from scratch.
This shirt from Charles Tyrwhitt is very close in color to what Sherlock was wearing. The buttons are different (the BBC shirt had buttons that matched the color of the shirt, while these buttons are white), but the shade is close to perfect. It's not in season right now, but odds are you can find a gently used one on ebay, and something similar will likely show up in their Fall clothing line soon.
This one from stylebop.co.uk is very similar. The shade is a little cooler than the BBC costume, but thr fit is right and the cut is very similar.
This shirt from Dolce and Gabbana is a different cut (there's a lot mkre rokm in the sleves), but the color is close, and a thrifty cosplayer could probably alter the cut of the shirt.
This is actually a shirt dress from Joe Fresh, which might be an interesting interpretation availible for genderswapped cosplayer.