The quickest option, short of someone on this list having an extra set, would be to contact a parts dealer (I suspect several would have them in stock). Several such dealers participate on this forum. Rocky Mountain Moggers have a list of parts dealers on their website (follow link on the Ultimate Unimog Sticky thread atop this discussion topic list, scroll down past various Unimog dealers lists).
I got one from ebay, but I suspect it was right time right place. I would try Scott at Expedition Imports. He has the 406/416 manuals, not much help if you only want the engine manual though.
As mentioned any of the dealers should be able to hook you up. However, I do not believe their is an "engine only" manual available. Normally you would have to purchase the complete workshop for a 406/416 or 1300 etc. If you are looking to save your $$, an adequate OM352 manual can be had online for the U.S. Military SEE tractors, which may or may not have the information specific to your engine. Most original manual roads will lead to BuchunBild in Germany...calculate out the cost, exchange, wire, shipping, brokerage, duty, etc., and I would be surprised if you saved more than $30-40. For most of us the manuals are a $$ loser/wash, offered more as a service to go with the rest of the product line.
For example:
125 EUR = $197.50 (give or take)
$30 Wire fee, or $6-7 paypal fee
Air Freight - $60-70
Total - $260-270
Can't speak for other dealers, but we retail for $305. About a 12% return on investment, not taking into account the damaged manuals, (crushed binders, etc.) that are written off as it is virtually impossible to claim on......