You just need to get a size fan that you think will suit your tank. Then you need a power supply. From there, you can make any number of stands or devices to place it over your tank.
For about $40, you can also buy ready-made kits on eBay that just clip on the rim of your tank.
I haven't put the fan rigs together yet just because it hasn't been warm enough. May do that this weekend if I get free time.