@babiak You can use the #Interrail "Rail Planner" app even without an Interrail ticket. It's not always completely up-to-date, so always double-check the actual times, but it can be good for planning.
Otherwise, bahn.de from #DB is the best route finder, even if they cannot always sell tickets for all routes, their route planner is very good and well updated.
Once you have found the trains and who operates them, it is usually cheaper to buy the ticket from the specific operator, but if you're getting connecting trains, buying a ticket from one source can help you in case of delays.