For the most part DS flash cart reviews are dead. Everything that is not the supercard, a R4i gold, which you will have to have to replace every other update for the 3ds it seems, is dead and nothing interesting is happening there. Nothing new will probably happen in them and even if it did you then face the "going up against the supercard" issue which already has a lot of homebrew, likely not easily ported out either, available for it.
Still the five ways to get things are
1) You pay for it yourself.
2) The site you work for pays for it. This can be like 1) if you own the site, you may have to do tax stuff though if you are being paid for it (UK self employment stuff is odd).
3) A vendor of such devices gives you one to review (you may or may not have to return it, for low value items like basic flash carts and mod chips you have to solder in this is less likely).
4) The manufacturer of such devices will send you a copy.
5) Write homebrew/software/fixes for the cart, this can also including being prominent in the scenes in question and this can also include being support for the teams in question if they have a forum (few do any more). Not so many people do this nowadays and most of the teams that ever did this are no longer doing it.
In the cases of 3 and 4 they will tend to expect a "? sent it to me" and it is considered good reviewer form to state that as well (also some kind of legal obligation in several countries I think).
1) is obvious, 2) is also obvious if you work for a site (and there are
not many that do flash carts these days).
For 3) vendors will have contact pages
For 4) manufacturers will typically have a website with a contact page.
In the case of 3 and 4 (and 2 for that matter) you will need to demonstrate you are not just looking for a handout. A previous body of work is helpful here, this need not be reviews and if you have good guides to setting things up, good articles on related topics (top homebrew or something) and other such things.
Consider also that your 10 hour review and nice link (and logo) to the vendor in question is probably worth far more than the shipping and bulk rate item they just gave you, especially as such a thing probably immediately went on their expenses.
Originally I would have suggested people people had a site*, I still do at some level unless you are staff on somewhere like this, but a good youtube channel and blog to cover it all works well.
*assuming you do not get a silly TLD a basic domain costs hardly anything (what £10 a year for a .com) and even if you just forward the domain and any emails sent to it right through to your blog/youtube channel and whatever email provider you use. If your would be registrar does not give you site and email forwarding for free then you have picked the wrong one.
Looking at those reviews, not bad. You probably want to a) get better lighting (have a search for daylight bulbs, they should run a little more fiver for one that works) and b) learn about white balance on your camera (or at least do it in gimp or something afterwards). Most of us are guilty of being caught short of on both of those but it is a simple fix. They are possibly a bit short though I already mentioned there is not a lot to look at in a modern DS flash cart review, your spelling and grammar seems to be fine at a glance which is a must (I gather the features section was original a list you neglected to remove the commas from). I am not sure where raw technical skill gets you nowadays, way back and for some systems you would be expected to have fair computer using abilities as well as the ability to solder but today it is not quite so relevant.
Again there is precious little happening in the DS world these days, precious little happening in the 3ds world either, which is why a lot of stuff you see around here as far as reviews go is leaning more towards home consoles (Devin did quite a bit not so long ago, both flashing chips and custom controllers), open source handhelds (though to be fair Another World has long liked those sorts of things) and more general news or features based.
That said if you enjoy doing it then you are not hurting anybody, I covered how to move up in the world there as well. Likewise if the 3ds scene does kick off in earnest (right now it is basically gateway and some software launched homebrew, most of which has not been released) you will be in a good position to point at your previous body of work when asking for a device that retails for £70.
