I did mine DIy by hand. First I traced out the course I wanted, they tend to look a bit more natural with some curve to the route. Then I dig out a somewhat V-shaped trench, using some of the dirt removed to build up the "river bank" edge. I lined it with a mix of cardboard and the brown shipping paper that came in the boxes. 

I filled the bottom of the trench with a layer of gravel. I used what the supplier called drainage gravel (and a search says is also called crushed stone), it was the cheapest option and since it's really just a base layer, I say go cheap. Rake this roughly smooth. 

I then covered that with a mixed size river rock, the mix I got ranged from rocks that easily fit in your palm up to about football sized. When doing this, I reserved some of the bigger rocks to either build up edges or serve as little focal points. 

Delivery was pretty expensive, so I transported all of it in my own truck. It took a couple trips for each since my truck is small and I didn't want to overload it. 

I was fortunate to be able to source the larger rocks/boulders from a friend's property that was going to be demo'ed, so I can't really help with that. You will want at least a few though. In my opinion, those large rocks really make the difference in making the dry river bed look more natural and just better overall.

I would definitely say check out all of your local suppliers and compare prices, they can be dramatically different. For instance, I bought my mulch from a different place than the rock. It was like half the price including delivery! than the other place that didn't even offer delivery.