I have booked a hotel in the center of Prague. Unfortunately they don't provide parking. What is the best possible option (cost) for parking the car downtown? Where can I get parking tickets (machine/shop/app)?
It depends on where exactly you want to park, as Prague is subdivided into multiple sectors with different price levels. Recently, they also introduced a zoned parking system: http://www.parkujvklidu.cz/prague-parking/
In short, this means that when in a zoned area, you can only park in the street in the purple zones for a maximum of 24 hours at a time, or in orange zones for up to three hours. You can buy a ticket in a machine or pay through the app (the cost is the same). You can also park in the blue zone for up to three hours, but in that case you have to use the app to pay.
If you won't be using your car daily and need to leave it parked for a longer time, you would have to use one of the commercial parking lots: http://www.parkujvklidu.cz/mapa-komercnich-hlidanych-parkovist-v-praze/
However, if cost is the main factor, it would be much cheaper to leave your car somewhere outside central Prague (for example on one of the P+R parking lots) and use public transport.
Park and Ride (P+R)
Car parks, usually located on the outskirts of Prague (though some are closer) with good access to public transport (similar to e.g., Vienna). These are the most affordable parking options in the city.
Cost: See the price list.
Capacity: They usually have hundreds (some of them almost a thousand) of parking spaces. You can check the number of currently unoccupied parking spaces.
Regular on-street parking and commercial parking lots
Regular on-street parking in and around the city center falls within Paid Parking Zones (PPZ), primarily intended for residents and local businesses. While parking is possible for non-residents, it is usually limited to a few hours each day and requires payment of a fee (through a physical or virtual parking meter). The fee varies depending on the zone's color, the district (sometimes even the specific street), and the day of the week. See the online parking meter.
Alternatively, commercial parking lots are available. These typically cost the most.