To start off attics make very poor growing areas due the extream temp flucuations, I see you live in Texas so temps this time of year can vary from cold to almost hot so outside air is not a good option. If the ceiling in this space is not insulated you may want to insulate it with R19 or greater insulation, then you may be able to cool the space with air from the other room. If there is a window in this space you may consider a window a/c unit but this will more than likely need a seperate power source then the wall outlet.

I hope this helps you and BOL.