We (228) have never hosted one, but the
Meriden Lions Club does one every year.
They use a large (about 15-20 foot wide) brook running through the park for the entirety of the race. However, because the brook has a lot of rocks, they have volunteers stationed throughout the race with sticks to make sure no ducks get jammed in stagnant pools or in small openings between rocks.
You also will need a lot of volunteers with waders and nets to catch renegade ducks trying to make a run for it after the finish line. Sometimes putting up construction netting across the river/brook will usually catch most of them. Also, it helps having a "funnel" type of thing at the finish line only big enough for a single duck to fit through at once. It will help you a lot in very close tie situations.
If you were to run through private property, you would need the approval of the property owners. Ducks
will get stuck midcourse, and even if the start and end are not on private property, you would have to trespass to get all the ducks.
As for ducks, if you buy them wholesale in a huge lot from an import company, they would probably be pretty cheap. After searching quick:
http://www.escoimports.com/shop/html...ber-Ducky.html
As ducks for $14.40 a case if you buy at least 24, and each case includes 48 3-1/2" rubber ducks. So if you bought 24 cases, that's $345.60 before tax and shipping, which would give you 1152 rubber ducks. If you "sold" a duck for $5 each, it would only take 70 ducks to break even. If you sold all, that's $5,760. But the best thing is that if you do it as an annual event, the ducks would essentially be free in future years. If you sell out of ducks, you can invest some of the profits into purchasing more ducks for future years.
I always had fun at the Meriden Lions Club duck races when I was a kid, so it's a great community outreach event as well.