HDD prices are coming down all the time, as are the capacities increasing.
Unless you use some special HDD's that are not readily available, the best thing to do is wait for a failure, then just pop out to your local PC shop and pickup a replacement.
If i was running a server without raid, and the whole system literally went down with a single failure, i would MAYBE keep a single drive lying around, but as we are on the same block as 2 pc shops i carry zero spares.