There are numerous Facebook groups that specialise in a housesitting in a particular country or region. There are even ones that focus on individual towns!
I've organised the main groups I've discovered into continents for ease of reading. But if a country you are interested in isn't listed, I suggest going to Facebook and type the word housesitting followed by the country, region or city/town you want. That will show you a set of results and you can then just click on 'Groups' to look at groups.
If there isn't a group, why not start one? During my research I noticed that many of the groups listed here were started by the same person. Clearly they are places that she visits - or wants to visit and has started the groups as a way of attracting people in that area to her.
A word of warning. Be very careful about committing to housesitting assignments sourced through these groups. Do your due diligence, take up references, have a video chat and get absolutely clarity ...