If you didn't get tickets in the Community Sale earlier in the year or didn't get enough, you have another opportunity as the RWC2015 Tickets are about to be released for general sale.
In order to get them you need to have an activated account, follow this link to create one Click Here. If you have already purchased from the Community Sale you can use the same account as before.
Then login between 10:00 on 12th September & 18:00 on 29th September when the sale goes live and purchase your tickets, note you will have to pay for them yourself and will need to use a debit/credit card.
This is a first come first served sale and you can purchase a maximum of 4 tickets per match, the earlier you login the better the chance of getting tickets at a price you're happy to pay.
Good luck!