We were in this situation when we bought. We had 2 cars and one space. What we did was to put up a sign on the strata noticeboard and also on the door to the garbage room asking if anyone wanted to lease us a space they were not using. We had replies and ended up leasing a space next to ours. This has worked out well as the owner is old, does not have a car and is happy to get some cash for it.
We are in NSW. Our solicitor told us that buying an extra space would be difficult as in our block they are all on the same title as the units and it would be very expensive and difficult to split up. He said it may affect the unit entitlements which work out levies etc which means all the owners would have to agree. In other words a big deal.