Taking a short position directly in real estate is impossible because it's not a fungible asset, so the only way to do it is to trade in its derivatives - Investment Fund Stock, indexes and commodities correlated to the real estate market (for example, materials related to construction).
It's hard to find those because real estate funds usually don't issue securities and rely on investment made directly with them. Another factor should be that those who actually do have issued securities aren't usually popular enough for dealers and Market Makers to invest in it, who make it possible to take a short position in exchange for some spread.
So what you can do is, you can go through all the existing real estate funds and find out if any of them has a broker that let's you short it, in other words which one of them has securities in the financial market you can buy or sell.
One other option is looking for real estate/property derivatives, like this particular example.
Personally, I would try to computationally find other securities that may in some way correlate with the real estate market, even if they look a bit far fetched to be related like commodities and stock from companies in construction and real estate management, etc. and trade those because these have in most of the cases more liquidity.
Hope this answers your question!