Which are most useful obviously depends a bit on the work you do, but by the sounds of it you're thinking mostly of furniture work in solid timbers, maybe with 'other work' as it arises.
Most chiselling can be done with a few firmers, or their more modern equivalent, the bevel-edged firmer. I don't think you really need many. Perhaps a couple of little ones, a couple of medium-sized ones and a big one - say 1/4", 3/8", 1/2", 3/4" and 1 1/4". For the very few jobs they can't do easily, a small selection of bevel-edged chisels with very fine lands - say 1/8", 1/4", 1/2" and 3/4". Then maybe a big, fat mortice chisel - 1/4" will cover most furniture-sized mortices. Anything else is either a bonus, or buy one if you find you really need it. Then maybe three or four 'beaters' you don't mind striking with a hammer and using in the garden for DIY and rough work.
Among the optionals - a small selection of out-cannel gouges (small, medium and large), a couple more sizes of mortice chisel, maybe a long paring chisel or two, maybe a pair of skew chisels (though for lapped dovetails, the 1/8" b/e will clear all the waste with just a small undercut covered when the joint is assembled), and maybe a drawer-lock chisel. Perhaps a few carving chisels and gouges of various sweeps if your work veers in that direction. Maybe two or three in-cannel scribing gouges. The optionals would give you a very comprehensive set of tools, with which you could tackle pretty much anything.
Sources? Vintage are nice, but getting really good ones of the right sizes may take a bit of searching, and you may accumulate a boxful of also-rans gtting them. The firmers could well be modern Marples from the DIY shed if you don't mind plastic handles. It's worth being selective about the fine bevel-edged (I'm biased, but I like the Ashley Iles ones - quite pricey, but you don't need many, and one purchase is for life). Proper mortice chisels will have to be vintage, but there are a lot of 1/4" ones about, and they don't cost the earth. Ditto the 'optionals' if you keep your eyes peeled for a few months, and buy very selectively.