You can have cheap, or good, but not both... LOL
The suggestions already made are good ones for "cheap"... If you want "good", well, it's gonna cost a little more...
Personally, I like a car jumper pack... virtually LIMITLESS supply of power (I powered the club controller one time off mine ALL DAY, and STILL had enough power to jump off a Suburban that someone left all the doors open all day on it and killed the battery dead as a hammer with the courtesy lights... They're not cheap, BUT, they do "double duty" by riding in your car all the time, ready for that inevitable day when you leave your lights on (and the car inexplicably doesn't turn them off itself) or the battery is on the way out, or someone asks you for a jump start (which is hard on your car's electronics). Plus it can power other stuff at a launch, which can be quite handy... They're also pretty much a "permanent solution" as you won't have to worry about launch batteries for a LONG time after you've got one...
If you don't want to make the "investment" that one of these will cost, there's cheaper options... RC vehicle electric packs make dandy power supplies... most RC battery packs are designed with a high "C-rating" which means the batteries have low internal resistance and can sustain high rates of discharge, putting out large amounts of amperage if needed. (Most RC electric motors in planes, cars, etc. take a LOT of amperage to run, ESPECIALLY "hop up" motors). They're pretty easy to find, and not TOO expensive, though more than what you're talking about. They also usually require a dedicated charger capable of charging their battery chemistry type (LiPo chargers, etc.)
Here's a potential "no cost" option-- if you have a battery-powered drill or other tool, you should be able to adapt the power pack to feed power to a launcher... Obviously you'll already have a charger, so that's no problem. You just have to figure out how to connect it up to the launcher. These can also deliver pretty good amounts of power, but not for as long as a jumper pack... but the cost is way lower too...
Anyway, that's some options to consider beyond the "taping a couple crappy lantern batteries together" routine... I mean, that's okay in the SHORT TERM, but if you're looking for a better solution, cheaper and "better" rarely go together...
Later and best of luck! OL JR