There is very little to no reason not to buy directly from Primary.
When I bought mine it came in really good time and they mark them up as "pneumatic parts for toy" so you don't get bashed by customs.
They're a stright up company.
If you wanted lesser products, Highpressureairsoft sometimes have the TAPP stuff in and there is a comapny called CT innovations based in EU (pfffft importing from US will be easier soon) but their adapters just aren't as nice or well finished as the Primary ones.
I've decided against getting one. If I hadn't have purchased 2 X WE Extended mags then I probably would have. But at over £100 + then buying the extra mags, I for once decided to just make do with what I've got.