You can check times at
bahn.de, although it won't sell tickets for this route - you'll have to go to
www.vr.fi/en/index.html to buy tickets.
VR's website seems to be working for me, although I haven't gone as far as finalising a ticket purchase - and as
Aviatrix has said, bookings only open 92 days in advance. (It seems VR's discounted fares are only available between 60-7 days in advance - although if you want to be sure of getting a sleeper on a specific day during the summer season, you may want to book it as soon as it comes on sale, rather than waiting in the hopes of a cheaper ticket and then finding it's all sold out).
Prices vary with demand, but it seems a ticket for a 2-berth sleeper, one-way is around €120 per person.