Are there any larger scale search engine type vendors like JA/Ritani to find matching side stones?
Would love to make a flanked ring with a lower cost setting vendor like DK but unsure if sourcing the actual side stones from him is the best bet?
If needed, DK will have stones custom cut to the exact specifications. His prices are very reasonable. I would let him source them.
What size are you talking?
As a rule, you gain quite a bit logistically by sourcing the stones, the mounting, and the labor from the same source. Aside from saving on shipping issues, it eliminates finger pointing if something goes wrong. If DK is going to build it, start there.