There are often small shops that resell tickets to movies, concerts, and train tickets. These shops are located near major train stations like Odawara Station.
Another way to buy tickets for sports and other events is online. One famous online shop is Ticket Pia.
I would be careful about using any other online ticket agency. They seem to be very expensive.
Some travel agencies in Japan sometimes sell concert tickets and others. There are many travel agencies in Odawara.
Japan Ice Hockey Tickets
The Asian League is the premier ice hockey league of Asia. Former NHL, KHL and Swedish league players are on each team as well as the best players from China, Korea, Japan and Russia. It`s great hockey! You can buy tickets here.
There are sometimes international games played in Japan as well. Check out Ticket Pia at the link above for those tickets.
Japan Baseball Tickets