Unfortunately, as has been established here several times, Spirit does not interline passengers to other airlines under any circumstances. (See here) Because of this, you are bound to wait for the next Spirit flight that can get you to your destination with availability. Upon checking the upcoming flights, there are no possibilities for the next week (even with a connection) to get you from IAH-LAS.

If it is urgent you get home, take the refund, book one of the several AA flights that are showing space today for $400, and consider it a lesson learned the hard way. I always have a backup when flying Spirit - like a later Southwest flight that I can change with no charge - and continue to just push that flight to match my next Spirit itinerary.