I love the links you guys did from Metzers, his site is a gold mine of information on raising waterfowl.
Dave Holderread (can never be sure I'm spelling his name right) is another gold mine of information. His website is good, however his book "Stories guide to Raising Ducks" is even better. I would say its a must read for anyone serious about raising waterfowl.
For ducks, Purina's Flock Raiser is very good, but it is always a good insurance policy to add Niacin especially for Pekins as they are the fastest growing duck and need those nutrients the most during their fast growth stage. Which is typically about 1 to 8 weeks old, after that they should be just fine on flock raiser alone. I'm planning to supplement for 10 weeks just to be sure.
All you really need is Niacin, which is easy to find at any Walmart, CVS, Walgreens, etc. Its also very inexpensive. Just make sure its not the slow release or flush free and you should be good.
The 5$ Vitamin / Mineral pack that Metzers offers for sale in their feed and supplement section is very good. (edit, just checked, its now $7) Its called BVS Poultry Starter Pak.
In addition to Niacin it has plenty of other vitamins and minerals that help your birds get off to a healthy start. Its only negative was they wanted as much for the shipping as they did for the supplement itself!
Being the tightwad that I am, I looked for something equal to it on Amazon so I could get free shipping. But alas, everything I found had significantly less Niacin, and often contained allot of salt. I'm currently using something I found at my local feed store but its not quite as good as the BVS stuff.
I did find this source, which still lists the product at $3.64
https://www.qcsupply.com/549992-bvs-starter-pak.html
Shipping is still an issue for me since their on the East side of the US and I'm in CA. But there you are, if you want it, that's the best price I could find.