The Chroma will come with two lens boards, one has a pinhole already fitted so you can shoot straight away, and the other will be blank so you can fit your own large format lens (shutter size selected following the campaign). The Chroma will accept most common focal length lenses from 65mm (with a recessed linhof/wista style lens board, not included) up to around 280mm. As with all cameras, the most important specification to be aware of is the 'flange depth' of the lens. This is the distance that the back of the lens (generally the lens board) has to be from the film plane/ground glass in order for you to be able to focus to infinity. If the lens cannot be set close enough to the film/ground glass, you will not be able to focus on distant objects (e.g. landscapes). The maximum bellows draw, or how far you can move the lens board away from the film, is 300mm so again, any lens that requires further than this to focus to infinity will not be suitable.
Personally, I would recommend a lens in the 'normal' range between 120-150mm focal length. These give a good general field of view and will allow the most amount of movement to learn how to make the most of your Chroma.
There are many places to view and purchase large format lenses. Ebay and other online market places will generally have a good selection and camera suppliers like FFordes/Real Camera Company/Mr Cad etc (UK) will always have a selection of lenses available.
I would recommend searching for photos taken at varying focal lengths on Flickr (make sure they are taken on large format as the field of view for the same focal length will be different for other formats) and see what lengths suit your own personal preference.