I suggest that you need to be a little more specific about what you mean by a portrait lens. "Portrait" covers a wide range, from a tight head-only shot, through head and shoulders and full length, to what we call environmental portraits, where a subject might be shown with the tools of their trade and the place where they ply it. (Photographer in the darkroom, farmer in the field, politician in the special place in hell.)
Each of these probably demands a different focal length.
But, with portraiture in particular, there arises the question of the way the lens renders the subject. Many people prefer a softer rendering of the skin and there are lenses which offer control of that. Simpler or older lenses may be preferred by some, to give a special character to their images. No doubt there are filters, too.
The standard, general-purpose focal length for 10x8 is 300mm. A longer focal length is often recommended for portraits, but depth of field becomes smaller, so apertures need to be smaller, too and exposure times may become inconveniently long for living subjects. You've probably thought of all this already, of course.
Best wishes for the project. I'm sure we'd like to see your results.
Depends what sort of Focal length you like for portraits, I tend to favour shorter than or normal for the format, however most like slightly longer.
In a shutter the most practical would be a 360mm Symmar, if you have a Sinar shutter then any barrel lens is feasible. The Cooke Portrait lenses are highly sought after so fetch high prices, the other issue is the ULF camera and larger sized wet plate users snap up the longer focal length lenses.
I have an un-named 20" f8 Rapid Rectilinear lens which I plan to use for portraits this covers 15"x122 it sits on my 12"x10" camera, I can also use it on my 10"x8" Agfa Ansco cameras and I have a large Thornton Pickard roller blind shutter that fits it or a single speed Gitzo stuio shutter with flash sync, Finding large enough shutters is difficult.
Another option is the convertible 240mm f5.6 Symmar (pre the Symmar S) it;s a slight wide angle used normally and just the rear cell is 420mm f11, it has the aperture scale marked for bot uses.
There's longer FL Xenars and Tessars but anything over 300mm is barrel mounted, but there's post WWII 360mm, 420mm, 480mm Xenars. There's also process lenses, I have a Bausch & Lomb Anastigmat f10 lens approx 450mm/18" to use with a 18x24cm Reisekamera. I'm lucky as I have quite a number of TP roller blind shutter enough larger ones to fit all my longer FL large format shutters. Better still is a Sinar shutter but they are getting rare and expensive (I don't mean the DB mount type). they give great versatility.
This is the 20" RR and shutter, for a sense of scale the shutter is 5"x6.5" and gives speed of 1/10 to 1/90th, it also has T. The smaller roller blind shutter is one of the more common smaller sizes. The large ones a few and far between.
Sinar lens boards are quite easy to find 10x8 DDS film holders are getting expensive typically well over £30-£40 each unless you're lucky. I picked up 10 for £70 at a Camera Fair 2 yeras ago and quite a few 5x4 for £2 each, but that's just luck. It's often much cheaper to buy them from the US but factor in the 33% Import Duty, VAT, and handling charge.
You need to decide your budget/options, a quick glance on ebay and lens 300mm/12" and over are becoming expensive and rarer in the UK.
From the US there's the Kodak 14" (355mm) f6.3 Ektar, you might find a 14" Dagor but prices will start at over £500 in a shutter, same goes for a 360mm Symmar, less for a Sinar DB one though.
Iy you go down the Process lens route or an older brass RR lens then I've never paid more than £20 - £120 for good optically clean lenses. The 20" RR was £120 and a 360mm f9 Apo Ronar was £20. Apo Ronars up to 360mm were also sold in shutters and better optimised for longer distances, also there's the G-Calons in barrels or shutters.
Final thought there's also the 450mm f9 Nikkor M, it's a small light modern MC Tessar type lens comes in a shutter, that's more of a typical portrait length lens equivalent to a 75mm on a 35mm camera.