In theory you can buy the tickets in open sale, but it is a pretty chancy business and is especially difficult if you don’t live in Japan. Fan club members usually get an opportunity to participate in pre-sale lottery for a chance to buy a ticket earlier. It’s not a guarantee but certainly is a much better chance for getting one.
Officially the fanclub membership requires a Japanese address but when it first opened there was an option to enter an overseas address and it seems this policy was unofficially kept when the FC management was transferred to Lawson. The newsletter is sent out four times a year and also usually there is a postcard for your birthday. Sometimes some small swag is bundled with the letter (last one tends to include a DVD with some documentary-style footage about what happened to Maaya during the year). There is an online fanclub shop where you can buy past newsletters and goods but they ship only to Japan (also, some goods are sold only at lives and not available in the shop afterwards).
I made a guide on how to register some time ago, not sure if it still works, but you can try it:viewtopic.php?f=6&t=692&p=12833#p12828
You can also find someone in Japan to register for you and participate in lotteries. Another option is various proxy services that offer a Japanese address and can forward mail for you. Some of them might even help you with entering lotteries and getting tickets, although with the current Lawson setup it’s less of a necessity since you can usually pick up your tickets in any Lawson once you arrive to Japan (but in some cases you may need to pay in advance and this may require someone to be physically in Japan). I’ve heard that some Japanese travel agents offer such services, so you can try checking with your local ones if you have any.
You can also usually find Maaya tickets on online auctions (especially Yahoo Auctions) but they tend to be much more expensive than the official prices (especially if paired or first rows). There are some online services that can bid and buy tickets for you but be prepared to pay hefty fees on top of the actual price.
There is a ticket exchange/sale topic on Maaya’s Mixi group and usually people who have tickets but are unable to go are giving up tickets for the standard price to fellow fans but this requires you to know Japanese and I think the exchange is usually done in person.
Me and a few others on this forum are in the fan club and few times in the past we managed to get tickets for other members so that may be possible too.