What can I use for coolant reservoir tank?

[bruceknee]

Harbor Freight portable air tank, $19, we have used them on the last 6 systems that we have installed

[belfert]

I use an expansion tank for my fresh water and for my webasto system. I paid less than $25 for each of them on ebay. Hard to build, convert or modify something for that little $.

Does the expansion tank you used have an inlet and outlet? 

I really would like something that can do both expansion tank and buffer tank.  Due to time constraints it is likely I will just buy an expansion tank with radiator cap from Sure Marine for about $150.  Yes, I could save $100 by doing it myself, but I just don't have the time to procure something and get extra nipples and such welded on.