This page explains how - I - have been obtaining my garage kits.
If you've bothered to come this far, I am sure you know what garage kits are. So, I won't bore you into explaining them.
Garage kits are hard to get a hold of in general. This is primarily due to a fact that most of garage kit makers are simple circles. They do not maintain their own manufacturing lines, nor are they interested in making massive profits.
They do sell their kits but usually to recoup their expanses and they usually sell their kits in events or festivals that they participate.
What all that mean is that anyone living outside of Japan will have an excruciatingly hard time getting a hold of such kits.
Of course, it is not helped by a fact that garage kits are a very niche hobby. It costs a fair amount to boot and failures are easy to come by, often ruining kits you spent a lot of efforts to acquire.
My method I am going to explain here is mainly focused on Girls und Panzer garage kits because those are what got me into garage kits.
Basically, I use a third party middle man to purchase kits in Japan and let them ship the kits to me. Now, I do pay a small fee for this service but their service has been proven invaluable in obtaining garage kits.
I use Neokyo as my middle man. Once registered, all you need is a link to what you want and they will figure out the rest, like how to get them. They will give you a total cost of acquiring your item in 24 hours and you must pay at this point to proceed further.
Once they have your item, they will send you an e-mail to let you know. Then you will decide which shipping method you will use and whether you want under-value your item to skip a customs tax.
In my experience, EMS will get my stuff in a week. Airmail is about 2 to 3 weeks. SAL is 2 weeks to 6 weeks. I really do not recommend SAL unless you don't really care about your shipment. Epacket is somewhere between airmail and SAL.
Now, I should have mentioned this earlier but this method will only work if the kit you are after is publicly available. Neokyo won't go out of their way to attend events/festivals to get kits for you.
What that means is that you need circles that are more than just circles. Some circles have an online presence and have set up shops at Amazon Japan. Some circles express their willingness to sell online if asked.
Amie-Grand is such a circle who have several important 1/16 scale kits for Girls und Panzer. They have a website and clearly express their desire to sell and even have an Amazon Japan shop page.
Sadly, Poly-Toys is a circle that exclusively sell in events only. However, Poly-Toys kits are easily found from GK which is a recast joint. Recast kits are copycats and probably illegal. However, given a fact that many kits GK sells aren't available online at all, I can't really blame people purchasing from GK. In fact I've purchased Poly-Toys recast kits from GK.
So, the bottom line is -
When you find a garage kit you really want, you will need to do your homework. Do the circle that makes the kit you want have a website? Does it say that they are willing to sell? Use Google translate to crudely understand what they are saying.
If they are willing to sell, you can use Neokyo and give them a URL of the kit you want and ask them to get it.
If above method fails, then you may want to find a recast kit instead. You can try GK or E2046.
Finally, I've had some luck getting garage kits from Mandrake online shop as well.
Good luck in your hunt because you are going to need it.