As long as the switch is rated to switch at LEAST 24 volts DC, at your rated current (15A), it would be fine. A switch which is rated at 15 amps will work fine at any amperage up to that point. You wouldn't need to get it oversized.
The only time that the current would EVER be that high is if the dc-dc converter output was at max current. Your operational load is going to be in the less-than 100 watts range.
As others have said, add a 15 Amp slow-blow fuse (or breaker) to the feed circuit. A slow-blow fuse handles short-term surge currents that you can experience when the converter starts up without blowing. That way, if you developed a short, the fuse would blow, the feed circuit would be disconnected, and nothing would fry.
And, as far as switch ratings go, that's the current that they are tested to handle correctly over thousands of on-off cycles. In an emergency, they can handle more current that that. (at higher than rated currents, switches can experience 'erosion' of the contact faces. One or twice at double the current shouldn't destroy the switch - but, it will shorten its life somewhat. If you repeatedly exceed the current rating, it would lead to a short switch life, though)
All fine and good, however, the switch is not going on a 15a line... It's going on a 30a+ line. I already have a switch after the DC-DC converter for the 12v stuff, it's the 24v 30a line going to the controller that I need a switch for.
The REAL problem I'm having is finding ANYWHERE that has any kind of selection of 24v switches. (I want a switch to turn on/off, not a relay.)