Where can I get, or how can I DIY table slides that will accept leaves totaling 104"?
Short answer, vertical and strong. I have an old Saginaw Expand-o-Matic that opens from 24" to  90" and is designed to only use the end legs, no middle support. Not sure where you could find this hardware today though.



You want the sliders to be vertical because the majority of the stress will be vertical stress of the table and contents weight. It also helps load balance because you'll need more than one slider to more than double your expansion and positioned vertically, your contact points are all on the same level.

An alternative is leafed ends, so the base/original table would remain the same but you could add on leaf ends, you can either have them hinged and permanently attached or be small stand alone tables that are add-ons. You'd need to make clean edges on the original table or route out the reverse of the rounded corners into the leaves/add-ons.