If I don't want to spend the time understanding the rail schedule, where is the best place to purchase a ticket?
What has your 'Rail pass in spain' to do with your question?
Surely you realise you must understand the rail schedules to know what timed journey so as to purchase a ticket.
What do you want from 'best'? Is this something to do with 'best' price or something else?
The train station? You question is very unclear.
Maybe read through MAN AT SEAT61