Most of us are familiar with Craigslist. On our first THS sit, we travelled by car from Canada to Arizona, with just a 3-week sit booked. While there, I posted ads in that city on Craigslist, as well as constantly checking THS, of course. Through Craigslist, we were able to book sits with two more families. Before booking, we went to the homes and met the owners and pets. After one of those sits, the owners signed up for THS. 
Many years ago I was looking for short-term accommodation in Britain. I came across Gumtree. Although my needs changed and I didn’t follow through with any ads, I found the site useful. I also thought the name was hilarious, and a little strange.
However, in Canada our equivalent, unique to Canada, is Kijiji. And to think I thought Gumtree was a funny name. 
If you know of a classified ad website, unique to a particular country, I encourage you to share it here with others.
