They have cornice mouldings at www.archpro.com (part of Outwater plastics www.outwater.com) I haven't tried these moldings myself but I have never been disapointed by this company (no affiliation).
Why are you so hot on foam mouldings? I have found that the most bang for the buck is at Home Despot. I buy the pre-primed, finger-jointed stuff which is extremely cheap (painted finish, not natural woodgrain). It shrinks a little more in the end-to-end dimension b/c of the various grain patterns but that is what calking is for.
It can be easy to mess up a piece of either wood or foam moulding so bite the bullet and get an extra piece, assuming you are going to mess up. If you use wood then cut it to fit, then sand and paint or stain and poly on some saw horses. Then nail it up and all you have to do is touch up the nail holes, no cutting in the paint.