You know about a couple years back we needed some for a car show. First off, I was completely joking around but I decided to ask CAL Trans. It's California's highway improvement slow poke workers. So I asked one guy and he gave them to me, we about a dozen, he said that they have thousands to choose from.
There is no offense to Cal Trans workers but it seems they take forever to get a freeway put up behind your house. Waking up at 6am every morinng to sound of jack hammer and guys talking about what to eat at lunch.
You may be able to rent some, but my guess is that you could borrow some from a local paving contractor or surveyor (I have 3 myself). If that doesn't work, you can order some from the surveying supply place that I use Hays Instrument Co.
They have 12" cones for 4.60 each, 18" cones for 6.97 each, or 28" cones for 11.86 each.