There are a variety of ways to get Mets tickets. Primary ticket sellers, like the Mets themselves or Ticketmaster.com often have some game tickets. Their prices are good, although it may be difficult to find the best seats.

Secondary ticket brokers are people whose business it is to buy up spare tickets and resell them for a higher prices. When they hawk them outside the stadium, they're called scalpers, but there are many brokers who run legitimate businesses from offices across the country. The price may be higher, but they typically have better seats.

I've bought tickets from brokers many times and always had a positive experience. Having brokers lets everyone get tickets if they are willing to pay for them, rather someone who manages to buy them way in advance. That's good for me, because I love the Mets so much, I prefer to pay more in order to sit right up close. Some of the better broker websites I've found are:
TicketLiquidator.com
Ebay.com
Stubhub.com
Hailstorm.com

Classified Ads:
Getting Mets tickets directly from a person can be risky, since they don't have the same security measures in place as there are for primary or secondary sellers. You still might get a good deal, especially just before the game happens. Good classified listings are:
Craigslist.org
USTBA.com

Conclusion:
If you just want to get in to the game, primary sellers(the Mets, Ticketmaster) are the way to go. For better seats and superior Mets tickets, broker sites like Ticket Liquidator are far more superior to StubHub. Enjoy the game!