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EDIT 4.4.07
GUNPLA INFO - THINGS LIKE MANUAL SCANS, RUNNER SHOTS, AND OTHER BASIC INFO ABOUT GUNPLA.
http://www.dalong.net - pictorial review of MGs HGUCs PGs and SEED kits sans the no-grade 1/144s. Also includes other Bandai kits like SDs, HY2Ms, HG mechanics, UCHGs, and Kotobukiya SRW kits. Don't have to understand Korean to understand this site. Has manual scans, runner shots, well, just check this site out if you wanna know what you're gonna get before buying a Gundam kit.
http://www.1999.co.jp/eng/gundam - Hey, it also includes runner shots and manual scans! No pictorial reviews of snapped kits, but pretty much like dalong, plus a wider selection of things to browse (its also a store

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Japanese sites
http://www.hlj.com - very good selection of kits, might be a tad expensive if shipping is factored in
http://www.1999.co.jp/eng/gundam - also a large selection of gunpla, their prices are 5% cheaper tha HLJs. has manual scans of kits if you wanna know what you're getting.
http://www.rainbowten.co.jp - 20% cheaper than HLJ. They do NOT have an online ordering system though (you email them what you want I think). I havent used this personally, but I read their staff is more responsive than the other 2 jp sites.
NA sites
http://www.gundamstoreandmore.com - cheap gunpla prices, (for an NA store) it takes 2 days for item to ship. PARTS ORDER AVAILABLE!
http://www.imageanime.com - this is as fast as gundamstoreandmore most of the time. They get the new stuff earlier than any other NA store. Price is a bit higher than other stores.
http://www.bigbadtoystore.com - generally expensive, but you might catch their gunpla sales (20-40% off). They pride in their 'mint condition' kits.
http://www.hobbywave.com - $10 flat-rate shipping plus very cheap prices. Shipping time is quite high.
http://www.hobbylinc.com - has alot of hobby tools and paints, but also a small inventory of Gunpla. Cheap prices
http://www.hobbyfan.com - Has Armored Core and SRW plastic kits. Resin recast kits can be found here
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Bad sites
http://www.gundamshop.com - reports that they do not ship orders and/or do not reply to calls or emails. Avoid
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Ebay (large ebay dealers) I have no experience with any of these, but I just look at their ratings.
Japanesetoywarehouse = 99.3% feedback rating
animetropolis = 99.2% feedback rating
tisinc99 = ~98%?