For people who are active in the SO community, it is quite common to run out of close votes. It is especially noticeable, when chat rooms like try to do some controlled cleanup in their particular areas.
The 50 close votes is just too low (and same actually goes for delvotes too).
The best way IMHO, would be for high-rep users to gain additional close votes based on helpful flags. This would mean that you have to already participate on managing the site, before you are entrusted with additional responsibilities. And would ensure that the additional ability to close bad question is not simple gained based on users urge for rep-whoring.
One vote per 100/200 flags, would seem a sensible rate (there is 50 flag cap per day). Preferably the effect should be applied starting from 500 flags (to counteract people who were just collecting for a badge).