Ticketmaster or become a member of your local Rugby Club as they have the biggest allocations!
Ticketmaster will have them, but I'm sure they will be so expensive it might not even be worth the time to check them out. I know there are discount sports ticket sites out there like Crowd Seats among others. They are the daily deals (like Groupon) site for sports tickets and offer big discounts without adding on those stupid ticket fees. Maybe they will be able to help.
Mate, go to the website or ticketmaster or the clubhouse, but jesus christ theyre expensive, ticket face value for wales is about 80 quid but i heard they were selling them somewhere for about 500 quid no exaggerations either. To be honest id get the best view and watch it on the telly, free, warm and best view of the game
Internet on the official website or go to the ground where the game will be held there'll be a ticket office there
