You can just email then that you're interested in XXX car, what's their best offer? Once you get answers from multiple dealerships, tell the others that XXX dealership offered $XXX, can they beat that?
I bought a new to me car a few months ago and found that most dealerships would let you walk away and I've heard that from several other people as well. Part of the issue is many dealerships were sending out letters wanting to buy peoples used vehicles because they didn't have the inventory to meet demand and now I think that because new car sales have risen they now have cars on the lot that they paid a premium for and aren't willing to take a loss....just my opinion.
Honestly, we weren't able to haggle. Most of the dealerships in our area are no longer commission based. They've set their prices a smidge lower to start, and won't budge more than $500 or so, and that's usually on your trade in, not the purchase price.