Where to buy Koyasan World Heritage Ticket?

I'd like to visit Koyasan, starting my journey in Kyoto in the morning and going from there to Koyasan without a stay in Osaka.

Can I buy the Koyasan World Heritage Ticket in Kyoto? If that's not possible, at which train or subway station on my way can I buy the Koyasan Ticket? (I'm planning to use a JR Pass for the trip from Kyoto to the Osaka region).

You buy it at the Nankai Station at which you will board the Nankai train. If you come from Kyoto, that will typically be Namba or Shin-Imamiya Station in Osaka.

Thank you very much!

Does anyone know the opening times of the ticket office at Namba Nankai Station? I'd like to buy the Koysan World Heritage Pass there either the evening before my trip to Koyasan or early in the morning just before the trip.

And does anyone know if there are large luggage coin lockers at Namba Nankai Station?

Thank you!

Does anyone know the opening times of the ticket office at Namba Nankai Station?

Generally about from the first to the last train of the day (4:30 to 0:20).

And does anyone know if there are large luggage coin lockers at Namba Nankai Station?

Yes, there are. It took me some time to find them, tough. It is a complicated station complex:
http://www.nankai.co.jp/traffic/station/namba/kounaizu_h.html

Uji, thank you very much for your help!

Depending on where you are coming from in Kyoto you will be transferring trains one, two, or even three times before you plan to store your luggage (assuming any storage is left).

Instead, you may want to consider staying at the same hotel in Kyoto before and after your night on Koyasan. They will probably let you leave your luggage there (mine did in Osaka) since you are returning. Then you won't have to haul your luggage around, and you can enjoy your stay on Koyasan more.

http://traveljapanblog.com/wordpress/2013/09/fudoin-%E4%B8%8D%E5%8B%95...

Thank you for your advice.

Unfortunately I will not return to Kyoto. I considered the luggage sending service, but after all, I decided to go from Kyoto to Namba already in the evening to avoid the rush hour in the morning around Osaka, buy my ticket, stay at a Namba hotel and have a look at dotonbori area and go to Koyasan the next morning. So I'll either leave my luggage in the hotel in Namba or in the station.