I have AMD cards with 6 outputs, but they are all Display Port.
I have not seen the other adapters for this type of thing because they are mostly for business use(Thus rather costly).
A board with lots of PCI-e ports would allow you to use multiple cards, but getting it all working may need custom software.TripleHead2go
was another device sold to allow one video port to control 3 screens. This was used by many user before things like AMD Eyefinity and Nvidia surround came out.