You have two options.
1) any cheap powerhead that'll get the job done.
2) get a powerful pump that'll mix the water and then allow you to pump it into the tank.
This website has some cheap powerheads that should work fine. CLL Pet Supplies
I'm actually getting one of their pumps through amazon next week (Aquarium 2400/3400/4800/6600 GPH Wavemaker Circulation Submersible Pump - CLL Pet Supplies [the 3400 gph version]). It's probably an overkill but it's still cheaper than the brand name stuff. (note, I would more likely go with brand names for in tank applications. But for mixing water...)
The Marineland maxijets are good and cheap, anything bigger than the 900 certainly isn't dead quiet so of noise matters to you keep that in mind. Another consideration is that a lot of people like to use a pump to remove water during their water change so something that can move a lot of water and handle some head pressure is nice. And finally what I always end up using is either pumps I have left over or as backup so you may with to get a backup for your main pump and use that.
Worth buying something you could use in the tank for a spare if anything breaks, so buy a powerhead or a return pump, whichever you might need if something breaks in the tank.