I tried a couple of the fans that are specifically made for strollers and felt like they just didn't put out enough air. This is what I ended up buying.
O2 Cool 5 Inch Battery Operated 5 Portable Clip Fan, it's about $20.
It's a bit bulky and it does have a large battery pack that you can attach to the clip. Keep in mind, there is a cord that attaches from the battery pack to the fan, my DS has never messed with it, but I also make sure it's not really within easy reach either. Takes several D batteries. I bought it in 2013, used it for several naps in the heat, a couple of trips to the zoo. I have not changed the batteries yet. I tried it the other day and it's still working fine with the batteries from last year in it.
It has a low and a high setting, low setting has been fine for most things.