Our contractor built ours using a hollow-core door as a base and applying tongue and groove boards on the face, some steel bands on the edge. It is beautiful! (Sorry I can't help with online sources.)

Thanks so much, Lori. That may be an option for us, depending on the trim guy our builder uses. We are very happy with our builder, but he seems to be more of a GC and less involved in building himself, if that makes any sense. We've checked our Real Sliding Door company online, and they're our leading contender for the moment.
Thanks again!

If you are building a new home and ordering doors, have the supplier make you slabs without the pre-hung option. As for an antique door, you could shop around salvage stores or have a painter or cabinet shop do the finish to a new slab. We used Real sliding door in the Prop style for this setup and were impressed overall with the quality and service. We have been having trouble with the plastic door protector pieces on the U-guides at the bottom. The peel and stick adhesive is not up to the task so we have been using 3M 77 spray adhesive for a better bond.

The door slabs are 3 panel Trustile in walnut.

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Here is a link that might be useful: Real sliding door prop

Brian- thank you very much. They look great!!