We use every possible app, service and website. You've got to really hunt around, to find good deals.
In this instance, we found the property on Air B&B about 3 years ago, we stayed here before, then you can compare on VRBO, craigslist, habitaclia.com, airbnb, realestate long term rental sites, etc...
Our current apartment is in a resort, it's like a ghost town really. So many units unsold, and so many that are owned and empty. So you have tons of leverage on a furnished rental. We are paying 350 euros/month for everything EXCEPT electric. Since heat and everything uses electricity, we are projecting about 150 euros a month. So a total of ~$500/mo for a 3 bedroom furnished apartment is great. Internet is 10 mbps/2 mbps, so not great, but also not horrible. It's enough to let the kids do their online schooling while I am still able to connect and get stuff done.
There are a number of resorts around Murcia with similar units on offer, search for the resorts like Covera, Condado de Alhama, etc... While you are there, you could spend 50k - 100k and pick up a unit yourself too.