I got Katie a Hurrta cooling vest.. It definitely made a difference in how long Katie was able to walk without panting. We usually walk at least a couple of miles each walk, but that's only mornings and evenings now because of the heat.
I purchased mine from Eurodesigndog.com. I use Havanese10 to get a 10% discount. Katja knows that Katie is a Yorkie, and she's very helpful.
Hurtta Cooling Vest
You can also get it at GW Little.
Dog Cooling Vest - Hurtta Pro Collection