Hundreds of places provide an aftermarket starter but there are many issues and I strongly recommend you buy the OEM Kit with instructions, brackets, template, and new cover from a reputable Briggs and Stratton dealer, such as us.
The part numbers over about 35 years are now superseded to 396306 for 391178, 243436, 243437, 326437, 394807. B&S now only shows 391178 as going to 396306 and the other part numbers are abandoned. In a price list 2003 from B&S I show 394807 additionally going to 396306. The series of numbers go back to 1973 or before.
When a company changes a part number, as you know, there is most likely a reason. AS such, the today part 396306 is actually a kit. The additional brackets, spacers, and instructions are part of the kit, not part of the aftermarket starters that proliferate from some offshore company who shipped a boat load. The kit has a few "IF...Then" instructions to accomodate earlier version applications. In addition, even if these mods are not requirred, the aftermarket starter unit gear fails nearly ever time it is mounted on any application.
Below are the included instructions for your information. I only expect a proper part with the OEM unit. To properly use the included template, size the image to fit an 8 1/2 x 11 sheet of paper.