One way is to collect the 3 gambling magazines and play the huge slot machine in Slot Ranch Casino until you have $2 million. Then buy the hummer from the Silver or Platinum strip and run over zombies. (i can't remember which strip is which but the hummer is at the North-eastern strip)
The gambling mags can be found in:
Ragazines - Royal Flush Plaza
Big Stan/earl's large print books - Palisades Mall
Shamrock Casino - One of the strips.
At the giant slot machine in Slot Ranch if you press "bet max" ($1000) you can win upto $100,000 at once, and with all 3 gamblings mags you have a better chance of winning.
The hummer keys are available for $2 million at a pawnshop.
But you could get the sportscar in Royal Flush for $500,000 or the chopper (bike) for $1 million.