I am just starting the planning stage of a high tech setup, reason I am mentioning this is cause I am a bit southwest of Milwaukee, and the ones you listed seem to lead more to the Northside of town. So while I plan, Ill do some pricing and edit in some south/ SW side sources as I find em.
Recently got a DIY Paintball CO2 kit from SnS through JaggedFury. I'm going to go ahead and recommend Paintball Dave's in the Third Ward. They were out of 20 oz tanks so the guy gave me a 24 oz for the same price of $24 filled. Refills are $2.
I have my tanks refilled at discount liquor on Oklahoma (1.25/lb). When it comes time for hydrostatic testing I swap out tanks at valley national gas (between collage and Rawson in an industrial area off 13th street).