There are many possible avenues. Which one is the best? It all depends on where you are and what level of the service you expect.
One of the biggest, and probably the oldest, directory for free coworking is Coworking Visa, with more than 450 spaces, offering free access with varied conditions. Coworking Visa allows active members of one coworking space to use other spaces around the world for free for a set number of days (3 is the default). Since the list is huge, you should always make sure in advance that the chosen space still accepts Coworking Visa. You may end up with a disappointing experience, like my ownwith one of the biggest coworking spaces in Berlin, where I arrived during my short trip, already behind schedule on my work. No one knew they were listed as a Coworking Visa partner, and I ended up paying 15 EUR, instead of using one of many other Berlin spaces for free. This is just an example of a bad luck and lack of planning, so you should definitely try Visa’s database and Coworking Wiki, of which Visa is only one part.
If you need something more unified, check Coworking Pass by Startuptravels.com. They have more than 30 coworking spaces all over the world, which offer 3 days for free. However, to use it you need to be an active member of Startuptravels.com, with the requirement of having acquired at least 3 new members for this platform (by sharing “your” registration link).
Last but not least, most coworking spaces offer at least one day’s free trial. Use copass.org, startupblink.com, desknear.me, or sharedesk.net, ask for your free coworking day. These websites work well in most countries, though they are not very accurate in Poland. letscowork.pl has probably the best coverage in Poland. Obviously, you will us in all the aforementioned places.