Matchbox has a vehicle that is just perfect for 15mm Sci-Fi games. Plan on spending about $1-$2 per vehicle. An excellent value all around. It has been produced under many names and comes in many different paint jobs. If you do a Google search for "Matchbox Armored Response Vehicle" or "Matchbox Armored Police Vehicle" you should be able to find lots of them. You may even be able to find them at a local toy store that sells Matchbox cars. The rear top of the vehicle slides backwards to reveal seats and a console.
Matchbox Armored Response Vehicle
Painted up and dry brushed. Gun was added from a VOR gun emplacement.
Gun and figure are from GZG's new SG15-A17 (NAC standing gunners)