In my High Res temp thread in the hydro section, someone suggested filling a 2 liter bottle with water, freezing it, and putting it in the res. I've done this before and while it does lower the temp, it only lowers it for like an hour or however long it takes the nice to melt. I suppose if you can't get and Ice Probe, you could freeze like two or three 2 liter bottles of frozen water and swap them out as they thaw. It would be pretty tedius but it might be your only option. The AC lowered my res temp to 75 so I will be able to ride that out for the last two weeks of my grow since the roots aren't rotting or anything.