Kester 186, or better yet, 186-18 (RMA with 36% & 18% solids respectively, which makes it easier to clean if desired/necessary). Unfortunately, the only small size sold are the disposable flux pens, which aren't very economical. A 1gal+ size could be split up if you've got enough people on board, and one willing to do the work.
MG Chemicals 835 RA liquid seems more practical for a hobbyist however (offer 125ml & 1L sizes, is reasonably priced, and easy to source in the US at least). Just a bit harder to clean IME (RA could be an issue on rare occasion for a particular application).