Try your local art supply stores - our local one has big 500ml, 750ml and 1litre bottles of acrylic paint for much less than the craft stores. The colour range is more limited than the little bottles, but for scenery use you can often get away with fewer colours.
I prime most of my scenery black before painting, so I have a 500ml black I've nearly emptied, and a 750ml burnt umber I originally got for Mars terrain that I've been using up slowly for years.
They sell white glue in big bottles too, very useful for scenery production.