I'd be extremely surprised if big-city tourist offices sell bus tickets; I'm not aware of that happening even in smaller places in Spain, though they're often good about providing schedule info. You can certainly buy tickets at the bus station, and quite likely at the station in other cities where the company has a presence. There are also many bus-ticket vending machines in Spanish bus stations, though I don't know that Comes has them. I've found the machines easy to use, with an English-language option. There has never been a problem with my US credit card. At least some of the machines have the option to print the schedule, which I find handy to have. I'll often print out the schedule soon after I arrive in town and tuck it into my wallet until I'm ready to book the outbound trip.
One quirk that's perhaps not obvious is that both machines and staffed windows stop selling tickets at an arbitrary time (5 minutes? 10 minutes?) before the bus is scheduled to depart. It doesn't matter whether you're the only one wanting to buy a ticket--if you're too late, you're out of luck (unless you can somehow work a deal with the driver, and doubt that would be possible). It's my strong impression that even if the bus is delayed substantially, the original purchase deadline still holds. I
It's never a good idea to wait till the last minute to buy a train or bus ticket. I once arrived 10 or 15 minutes before departure only to discover a system outage made all the vending machines unusable. Naturally, there were substantial lines at all the ticket windows. I was very lucky to get through the line in time. You never know when the person in front of you wants to buy six tickets on a convoluted itinerary.
Seville has two bus stations. The Comes buses to Cadiz use the one at Prado de San Sesbastian.