I got my coil from O-reilly's & Advance Auto it was BWD E5 paid around $20 for them. have the same coil in my 1929 and 1930, and recently put one in my friends 1931. and i even bought a spare, granted not all at the same time.

if you keep killing coils that quick i would be looking at other things that could be causing it.

forgot to add: both stores have a limited lifetime warranty on the coils :)