Going to be making a frame and attaching buckets to it so my kids can practice their casting (me as well). Where can I find casting plugs? I had googled it and came up with nothing.

Try Bass Pro, I was in one of the stores last week and saw a few on the racks.

I was there Sunday and didn't see any. They may have been out of stock. Should be going back next week. I will look again. I tried a local place and when I asked the guy he had no clue about what I was talking about.

This will give you the item #, or you can just print out the page and bring it with you. I have a couple different styles, and these are my favorite by far. They're made of rubber, so they just bounce when/if you cast them into the road...which I do all the time. 

http://www.basspro.com/webapp/wcs/stores/servlet/Product_10151_-1_10001_13377____SearchResults

why not just take an old jig with a rusty hook - cut of the point and barb and let 'em throw "real" lures?

My son (he's 5 yrs old) learned a baitcaster on a hookless crankbait this year.

lead sinkers work too...

From my experience, throwing casting plugs is very different than throwing a t-rigged plastic... What I do, and what I would recommend, is practice with baits that you will be using on the water.  I would compare practice plugs to a basketball player who practices jump shots with a baseball... After all that practice with a plug, when you get on the water with your bait, it will be totally different.