>>13763819
Gunpla general, as much of a mess as it is now, has a guide outlining exactly this sort of thing. Please use it.

Gundam Planet
HLJ
AmiAmi
Kirinhobby

Take your pick

Amazon is pretty decent though pay attention to the seller.
You can get an MG for probably 15-25 dollars cheaper than at a local shop, but sometimes it takes about a week or 2 in shipment

Location, friendo? Depending on where you are, you might have a local (or at least in the same country) supplier/importer. In my experience they can be more pricey than if you put in the legwork yourself; however, you will probably have an easier time organising returns or resolving disputes than say if you order through Hobby Link Japan or AmiAmi.

Ignoring location, Amazon and eBay are pretty solid routes if you don't want to go through other sites. Just be sure to check seller feedback and such.

Oh, in the future look for the Gunpla general thread. If it's not on the first couple of pages then it's just a search away and has more info than you could ever need.

>>13763819
I used to go to Yodobashi Camera :'( Now I stick to HLJ for MG's because I can leave stuff in the "Private Warehouse" and just stack stuff up over a couple of weeks then ship it all at once. Plus MG's are stupidly over priced on Amazon and at local hobby shops. But for HG's and RG's you'd be surprised what you can get on Amazon. Prices and shipping options go up and down on a daily basis, so if you put a kit into your cart then check it every couple of days you can snag some good deals. I would get HG's for under $20 and RG's for about $28 and with 2 day Prime shipping.

Gay