I am looking for a 24 x 36 oval mirror front medicine cabinet that will insert into wall above a vanity sink countertop

There are many on the internet https://www.google.com/#hl=en&gs_rn=...w=1675&bih=824 but you have not told us the dimensions of the wall cavity

24x36 would be a strange size for the box. Mirror perhaps. Even with a house built on 24" centers, the wall opening would be 22 1/2".

I have recently installed medicine cabinets but can't find the exact match for you, Try this link: Natural Oak 24x30" Tri-View Medicine Cab. Mirror

Look up online sites through a Google search. Look for sites where you can get customized cabinets, "made to order" that can give you exactly what you're looking for.