You can get an extra 29W wall charger direct from Apple, I believe it is $49 in the US.
I know you are price sensitive, though, so if you've got an iPad Air or newer wall charger already, you can use that to charge as well, then you just need to get the USB-C to USB-A cable. You can get a charging only variant (like the one the rMB ships with) pretty cheaply if you look around a bit.
If you are talking about an external battery pack, then you would be best served following the advice above and go with some known to work well, like the Anker products mentioned.