(on Ebay and/or from the used boat parts marketplace). When old manifolds wear out or freeze, they're disgarded but the temp senders are saved and resold.
As Ernie noted, I'd check with Dan Cook first, because I'd prefer to have a new one in the box. If he doesn't have them, Ernie has a couple good alternatives noted, but you can also check with Hundley in Texas, and he probably has a few dozen sitting around.
http://www.hundleychriscraft.com/
ABOVE: 327F manifold on the left, early 283 manifold on the right with temp sensor.
Regards,
Paul