The amount of other materials present would make it undesirable. Alot of catalytic converters dont even have much palladium in them;
You would obviously want a brand new one, that was clean and easier to extract. So why spend all that money on a catalytic converter just to extract palladium that can be bought much cheaper than a catalytic converter that may be skimping on costs by substituting the expensive palladium with other metals such as Cerium, Iron, Magnesium etc ect. You would need an expensive one as it would have the most palladium in it.
eBay will bring up sources of palladium which can be converted (more easily than extracting the palladium from a catalytic converter) with HCl and chlorine to produce palladium chloride. Or just buy palladium chloride? there are a few places I can find selling it.
from wiki;
"Palladium on carbon is commercially available, with a CAS number of 7440-05-3. It can also be prepared in the laboratory. In a typical procedure, palladium(II) chloride and hydrochloric acid are added to nitric acid-washed activated carbon. This composite is then dried, and the palladium(II) is reduced to palladium(0) with hydrogen gas, and washed. The palladium loading is typically between 5% and 10%.[2]"
The only hard part is effectively reducing the palladium (II) to (0) by hydrogenation, but I would rather generate hydrogen over HCl anyday! And also it can be stored as palladium(II) safely, and when ready to use it, then reduce it to palladium(0) and when it has the absorbed hydrogen it is somewhat more dangerous to handle.
note; Many sources of OTC palladium coins and ingots etc/
PALLADIUM CATALYSTSOrganic Syntheses, Coll. Vol. 3, p.685 (1955); Vol. 26, p.77 (1946).
http://www.orgsyn.org/OrgSyn/default.asp?formgroup=quick_form_group&dataaction=db&dbname=orgsyn