I live in Orlando, FL, where I recently found inexpensive plastic toy vehicles that are compatible in scale with Minimates:
Dollar General sells a $3.00 "bobcat" style "no-cab" bulldozer that seats 1 figure as well as some $1.50 boats
WalMart offers land vehicles, aircraft, and watercarft that can seat various sizes of figures, including Minimates, ranging in price from $1.68 to $2.99
Walgreens Pharmacies offer small plastic "cigarette-style" boats that can seat several Minimates; they usally cost $1.99 each, but this week, you can get 3 for the price of 2, $3.98. In late summer, the price has dropped to $1.49 to clear the inventory for the fall toys. They also have other types of watercraft at the same prices that would work as well.
In all of these cases, the vehicles tend to be very simple, with a single-color top over a white body; you can try to customize them with decals and/or paint that works on plastic.
I was browsing a local Asian Dollar store, and found a $5.00 "Concept car" that has lights, sounds, and "morphs" into a larger vehicle. I got the last one in stock for $3.00, and it seats Minimates fairly easily.
Has anyone found Minimate-scale furniture, buildings, vehicles. etc.??? If so, please post a reply, and help us all out.