How will you build Gunpla if you don't know where to buy them?! Here are a few options for finding the kits you want!
Probably the most popular place to buy Gunpla is online. You can find almost any kit you want on the internet. The problem is shipping! Because many Gunpla kits will come to you straight from Japan, shipping can be very expensive. You may even need to use a proxy service.
Getting Gunpla straight from Bandai's website is a good idea. Bandai manufactures every Gunpla kit, so they have a lot of variety. Other sites such as hobby link Japan are great too; Bandai isn't the only place with Gunpla.
Local stores are also good! Prices might be high, but at least you're avoiding shipping costs. There are probably more stores carrying Gunpla in your area than you realize! It's always worth a shot to see if they have anything good.