when I bought my '07 vert last year, I had the same concerns and ultimately found what I wanted by getting a CPO off-lease from a BMW dealer out of state (and BMW NA extended the free maintenance out to 6 yrs/100k miles as part of a sales program incentive). To me, it was well worth the somewhat higher price of the CPO over private party/indy dealer prices. Not sure about third party extended warranties, but presumably you can buy them on the sixes just like any other kind of insurance - just make sure you go over the fine print exclusions very carefully.
I noticed you are in Atlanta - if you're interested, send me a pm and I'll share my thoughts on the Atlanta dealers and who I ended up buying from.
'07 650i vert, beige/jet black, sports & cold weather pkgs, comfort access, logic 7, sat radio, Miro M6 wheels (with Michelin super sports), M6 spare tire, tint, windscreen, triple resonator delete, m-tec v3 AE's bulbs, corner bulb delete and m-tec fogs
'11 Acura MDX Advance with Entertainment pkg for wife - no mod's
'04 Acura TSX (daughter) and '12 VW GTI - Autobahn (son)