According to the Travel Service Website charter flights are seasonal and are provided as a service to travel agencies:
On its charter flights Travel Service transports not only clients of Czech and foreign travel agencies
So the only people who would know about these flights are the agencies or an agency that charters it. The fact that flight comes and goes with some regularity there is no requirement for Travel Service to make this flight if it has not been paid for.
Namely: Air Charter service is renting out the entire aircraft (and crew) to an agency which then sells the tickets usually as part of a package case in point USA3000 airline which had a regularly scheduled flights to the Caribbean islands from the US but after 2008 you could not book the seat on it individually only from Apple Vacations as part of your vacation package.
So if you want to purchase a seat on a chartered flight you will need to get in touch with the company that chartered the plane and even then they don't have to sell you a seat only if they are selling seats as part of a package.
P.S. The closest alternative available for getting a seat on a chartered flight that I had found would be Jump Seat. This doesn't mean that you will get the flight you want or even that someone will want to deal with you if you found.