Your best bets would probably be Amazon.com or Barnes and Noble's website. One way to know is that usually it will say it has English language tracks in addition to being NTSC formatted and Region 1, and on Amazon a thing to make sure is that you buy from Amazon, and not one of the sellers, unless you are absolutely sure of what you're ordering. I've seen sellers on there hawking bootlegs in the past.
A general rule of thumb is, if the price is too good to be true, it's probably a bootleg (others are that if there are 7 or 8 episodes on each disc as opposed to 3 or 4, or too few discs per season, if the discs are Region-Free/Region 0, if it says it's imported from China or Taiwan, or if it has Chinese subtitles and English audio/subtitles).
As an aside, legit, imported single Japanese DVDs will be in the $50-100 range; boxsets will usually be well over $200 - this is because Japanese retail is typically higher than the American equivalent, and even when it is the roughly the same (like the Evangelion Renewal series discs, not the movies), the most secure and fastest type of shipping (in the U.S., this would be the EMS service) charges $18+. If something is touted as imported yet costs $15-25, it's almost certainly not legit. Legit Japanese discs are also Region 2, as opposed to the U.S. and Canada which use Region 1. The thing that sets it apart from Europe, though (which is also Region 2) is that Japan uses the same video format as the U.S. and Canada, namely NTSC, whereas most of Europe (and most of the rest of West and South-east Asia, Australia and Oceania, Africa, and Eastern South America) use PAL (I also think French Canada uses both NTSC and PAL). A chart is available
here
The average price for a single legit DVD is usually $24-30 (sometimes you can find marked-down or older releases for $18-22), and boxsets usually range in price between $80-120; $120 sets are typically full-size DVD cases, and usually reserved for 5-7 discs in the set; $80 sets are typically full-size if there are say, 4 discs in the set (like in the case of Hellsing), or thinpack if there are 6-7 discs in the set (like Mahoromatic or RahXephon). Rarely, you'll find thinpacked or clamshell sets for $40-60, like Love Hina, Please Teacher!, the old release of Generator Gawl, etc. (in that example, Love Hina is 7 discs, I think, and Please Teacher! and Generator Gawl both have 4 discs - Gawl was $45 at Best Buy, if I remember correctly; this of course, was also back in 2003 and it's since been rereleased).
As to whether a possible purchase will come with the whole series or not, it depends on A) if there are a standard 13, 24, or 26 episodes in the series (in other words, if it's a single run or if there are multiple seasons), B) if in the description it says whether it's just the first disc but comes with a commemrative box (which is actually a smart buy if you're going to be buying the subsequent discs as they come out).
In the case of Gundam Seed Destiny, this is the page on Amazon:
http://www.amazon.com/gp/product/B0...4595648-5351931?_encoding=UTF8&v=glance&n=130
And that says that it's simply the first volume with the series box. None of it has been released yet, it would seem.
