Congratulations John. I hope you're as happy with your Green Monster as I've been with mine. It's an amazingly capable camera, considering the price that they go for.
There are plenty of places to get film holders on line...I'd start with Midwest. Give Jim a call and see what he has in stock.
I prefer wooden holders, both for their weight and their durability. There are a lot from the mid-20th century floating around...various manufacturers, generally painted black. Most of the holders I have needed little work. Just some new tape so the hinge is firmly attached and a quick test for light tightness, and you're on your way.
As for lenses, I'm sure you're aware of the usual recommendations. I do mostly slow film, long exposure work, and I've found that barrel mounted process lenses give me the best bang for my buck. There are plenty of them out there, and some of them throw a pretty amazing image circle. For portraiture, my first thought is a Nikon 480mm f/9 process lens. I have mine mounted in front of a Packard shutter with the instantaneous setting. The instantaneous setting is useful for portrait work, and holding the lens open with the bulb works very well for landscapes.
I'm working up a lens board/shutter frame arrangement so that I can mount any of my lenses in front of the same shutter...it's easy to design but I haven't had the time to actually build it yet. The only real issue was dealing with lenses with differing back element distances behind the lens board, but I'm going to rig it up so I can slide the shutter forward and backward when I change lenses. It should be easy enough, if I ever get the time.
Be well.
Dave