well, for starters if you have a case that can take two power supplies at one time, you can install two power supplies, and pick and choose which one gives power to which, however, this is an overkill. i'd say just get a whole new psu, which will have enough fan connectors, and be more up to date so you can upgrade other components without worrying.
but if you really must get another psu for just the fans, and you dont have a large enough case, you can get powersupplies that fit into 5.25" bays.