I ordered a 5ltr bottle from amazon cheap. Also worth mentioning that I've seen a few recommendations for those cheap squeeze bottles with a nozzle to control the fluid a bit better. The bottle that comes with the unit looks to be an open topped bottle.
If you have any recommendations for bottles I would be interested. I've been using a syringe that sort of works, but it has started to stick. I always out on less than recommended and still end up getting solution everywhere