DS Flashcart R4 vendors

First off, whether or not that site is legit, those prices are too high. You can get them much cheaper nowadays.
Just an FYI, the original R4 makers disappeared and stopped doing updates a long time ago due to legal issues and 70% or more of anything that says R4 nowadays is a cheaply made imitation or just a complete scam. There's also the fact that most flash carts made for DSlite won't work on newer DSi/3DS systems any more. If you've got a DSi or a 3DS and would like better features like better quality media players, savestates, multitasking while in-game, or GBA emulation(!), I suggest checking out the "what flash cart should I get?" thread on GBAtemp.
All that being said, if you just need something simple just to run DS games and not much else in terms of the extra features I mentioned, as long as you're careful with it an R4 clone will do just fine. I have an original/official R4, and two clones (among others) just for running extra copies of KORG DS-10, KORG M01, and Jam Sessions on them, and they're fine. One other thing to make note of, though, is that M01 (and pretty much any other games released since late '08) won't run on the original R4 firmware since it's so outdated. A guy from GBAtemp started making his own firmware for the card, though, and I've hardly had any issues with it. It's called Wood R4. Look it up.
Anyway, I'll go ahead and link you to Shoptemp. (sister site of gbatemp)
The most well known and reliable flash cart sites are all listed on there by what they sell and what the prices are like.
There's even a section just for R4 clones. I paid like $2 each for mine with $2-3 more for shipping.
Some of the cheaper sites seem kind of sketchy due to the bad engrish and poorly made websites, but I can assure you that they'll be legit. I've ordered a lot. Shipping is often hella slow (IIRC one time I had to wait 5 weeks for one of my carts to get to me), but I can't really complain since it was such a great price and I wasn't in a rush to begin with.

for reals try craigslist. theres usually someone selling one for cheap. at least there is where i live. also if youve got the cash id go with a cycloDS before an r4 but thats just me.

tbqh if you've got the cash at this point you should skip the slightly older stuff like acekard and cyclo and just go for a Supercard DSTWO since it's still being updated and works on all DS/DSi/3DS versions at this point.
IIRC Cyclo's been abandoned for a while too.

@PianoGameboy, Thanks for all the help! So pretty much any of the links from ShopTemp should be pretty safe?