I just realized while I did link to some places to buy Gundam in previous entry, I didn't specifically tell you what online sites are great for buying Gundam when you do not have a local place to buy kits. I myself have not bought a kit since my local shop temporarily closed (they're opening back up in March so I'm sure I'll be getting my Gundam buying fix soon after). You can also get kits on sites like Ebay, Amazon and any other bidding site but I'm going to show you official sites.
HobbyLinkJapan is really good if you want to be able to preorder kits and buy directly from Japan without having to go through a 3rd party service. The prices are very good and the kits are packaged very well when being shipped- basically they come in tact and if you have a problem you can contact customer service. HLJ is very good about fixing problems such as a kit getting squished. I put this site first because they are good for more than just Gundam kits- you can get shirts, supplies and other things that aren't Gundam related.
You can get more than just Gundam kits here. There are even ones of the ship from One Piece. I myself have never used this site so I don't know if they ship their products well but I have had people recommend I order from here... so maybe one day I will!
The website promises a "110%" savings on price. That doesn't mean you get things for free though (as cool as that would be), but it just means they are set on giving you a good price which it seems they do. I have a friend who has build over 100 Gundam kits and he's bought a pretty good amount from here.
HobbyWave is one of the more well known sites (after HLJ) to buy Gundam, but the advantage is this site is mainly Gundam with a few other series and divideds the products by grade and scale which is pretty convenient when you aren't ready for the high grades. I've gone to websites and seen really nice looking kits only to find out they were perfect grades or master grades when I had just started building Gundam and would get sad knowing my skill level was no where near where it needed to be for PG/MG. It's nice not getting my hopes up.
J-list is not the best for actual kits (I did a search today and did not find any but sometimes they do have kits listed) but for the magazine's that either focus exclusively on Gundam or feature Gundam. You can also get things like this figure: