K&S metals is your best bet - they have a lot of stock sizes of brass tubing, and offer two sizes of "oval tubing" - neither of which quite meet your specs.
https://ksmetals.com/pages/tubing-material-selection-pageBut fear not, you could:
a. buy round tube of the right size (bit of calculating required here) and run it between rollers to flatten it to get the 3/16" width, then mill or slit the tube to get the open size you want. Probably do an anneal operation before cutting so it won't spring out so much.
b. forgo the above experimentation, and use the "Custom Jobs" tab on K&S metals website to order a custom run in the size you need.
Good luck!