I am getting ready to run in to the same problem myself. If the room is in a house that has a crawl space beneath it that is enclosed, you can draw air from there to pull through the grow. Use a strong blower that will exchange the air in the cubic space at least once a minute. The air from under a house is always very cool in the summer. I checked under my house once last summer when it was tapping at 100f and under the house was 73f. I was amazed but the ground is always cool.
If you reall want to do it right, you could dig a large hole under the house that would pool cool air, then exaust all the hot air out as high as possible.
However, if you are in a situation like me where you don't have a cool crawlspace to pull from, you can make your own AC with a couple coolers, 2 auto AC evap coils, a strong pump, hoses, a fan, and some dry ice. You start by setting up one AC coil in the area where you need cooling. Connect the hoses to it and route one of them to one of the water coolers with the pump and lots of water in it. The other hose would go outside the grow area and attach to the other AC coil. One hose would run back from that coil to the cooler with the pump.
You could either seal the system or have it pull from the pool of water in the cooler inside and then dump back into the same cooler. This cooler will be filled with the coolest of water and then have a small amount of dry ice added to it. This will cool the water and release CO2 into your grow room. the cold water will be pumped(with relatively low pressure) into the first coil that has a fan blowing on it so that the air is cooled(this is within the grow space or in the space from which the intake air is being pulled). the water will then continue on to the outside to another coil that is sitting in a cooler of cold water. This will pre cool the water before it is returned to the cold holding cooler with the dryice.
This is a "red-neck" AC idea that I have recently come up with. It is as yet untried but I feel like it would work for a couple days until the dry ice has evaporated and the cooler water has warmed. For the hottest days the expense for the dry ice might be worth it. I haven't worked out all the details.