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=11&gs_ri=psy-ab&tok=f5LPZH0ydP1KJoIupsXsVw&cp=7&gs_id=q&xhr=t&q=oval+medicine+cabinet&es_nrs=true&pf=p&output=search&sclient=psy-ab&oq=oval+me&gs_l=&pbx=1&bav=on.2,or.r_qf.&bvm=bv.45645796,d.aWM&fp=7e3d79c053af5d56&biw=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 (http://www.thertastore.com/Natural_Oak_24x30_Tri_View_Medicine_Cab_Mirror_p/gtrv2430l-horning.htm)

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.