- Where can I purchase the 20 minutes time tickets for public transport in Warsaw?
- Can I get it in all of the Metro stations and in Bus? Can I pay by credit card?
- Can I buy it using vending machines?
You can buy them on-line (but you have to have Warsaw City Card bought in advance) or in one of the sale points, including post offices, service center, kiosks, ticket machines. Generally you can buy them on every corner, especially if you come by train to the city center.
There is a detailed search available on ZTM's (the public transport company in Warsaw) website in English. You can filter vending points that accept credit cards and sell time tickets. There's ~300 of them, you can look for individual stops if you're interested.
In the vehicles you can usually get it, but not always, and if the machine is broken the regulation says you're obliged to have a ticket anyway:
Ticket machines in vehicles
Zarząd Transportu Miejskiego develops a ticket machine network. At the time it includes all buses, trams and trains delivered since the year 2009. Soon ticket machines will appear in following vehicles. The Public Transport Authority gradually develops a network of ticket vending machines on board of its vehicles. As for now they have been installed on board of all SKM trains, in new trams and many buses, including vehicles operating on line 175 which runs to the Chopin Airport. Remember! If the passenger is unable to buy a ticket from a ticket vending machine or from the vehicle staff he/she is not exempted from the liability for traveling without a valid ticket. Ticket sales on board of vehicles is only complementary service to the fixed network of ticket sale points.