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 mostly around the outskirts of Prague (however some of them are closer) with good access to public transport (similar to e.g. Vienna) are the cheapest and convenient method of parking in the city for less than 7 days.
Costs around: €1/day + extra €4/night.
Capacity: They usually have hundreds (even thousands) of parking spots.
Tip: You can check the number of currently unoccupied parking spots here.
Regular on-street parking and commercial parking lots
Regular on-street parking lots in and around the city center are considered PPZ (paid parking zones) that are basically meant for residents and local businesses. You could still park there, but then you would have to consider the color of a zone, district (even street) or day of the week (to know how much you have to pay and if you are allowed to at all). And even then it would be for couple of hours at max. On the other hand you are free to use commercial parking lots, they usually cost like €20/day.
Go to google maps and search for parking. By prepared that its 500m away from your Hotel. Be prepared to pay ~25 USD a day.