Aside from fabricating them, the best place would be another Seneca that isn't in good condition, but has its hardware.
The next best alternative would be a parts camera from another maker, like a Korona or perhaps a Kodak 2D. The metal brackets on all of them are similar, but won't be exactly the same. Finding a suitable parts camera could take some time.
You could also try Richard Ritter, he does a lot of LF repair work and may have pieces or a parts donor available.
Note that even if you find Seneca parts, you may need to relocate the pins on the back that engage your missing clips once you find suitable ones. I had to do that with a Korona that I repaired and married a back and camera body from different vintages.