Seems like you're planning for a 55 gallon tank.
Buy the biggest tank that will still fit wherever you want it to fit. For my two 10 gallons and 7 gallon cube, I have a manifold coming from a 15# tank. If you can get one, get a 20#.
Tanks would most likely be cheap locally or from a site like Kegerators, Wine Refrigerators, Water Coolers, Beer Taps, Compact Refrigerators | BeverageFactory.com
. You can't ship full tanks, and they're pretty heavy empty anyways. Look at local brewing or industrial gas supply/welding shops.
As for the regulator, if you want cheap, you can try the Azoo regulator from DrFostersandSmith, or JBJ or Milwaukee regulators from eBay. Check the forums on here too, sometimes people sell theirs pretty cheap on PT. These regulators come with a needle valve, and the JBJ/Milwakuee come with bubble counters.
As for the tubing, get it from wherever you get your regulator (ie: some eBay sellers will let you buy tubing from them too). If not, get some from Rex Grigg at Rex's Guide to Planted Tanks
. Just look under the CO2 stuff for sale tabs.
Filled 20# tank = ~$95-110
New regulator = $90 shipped
Tubing = $10
~$210 for new stuff.
I pieced my setup together slowly and ended up paying $196.50 for a 15#, a used JBJ regulator, a 3-way manifold with 3 bubble counters, and 3 nano glass diffusers for my 3 tanks. Took me maybe a month, month and a half to do that though.