MSA = Motorsport Auto.
I.e. The biggest Z parts supplier in the US and source for a lot of Mike's parts (that's who he is talking about when placing bulk orders from the US).
There are then a variety of other businesses over there dealing in more niche and tuning parts. Z Car Depot, ZCCJDM, Whitehead Performance, various other eBay sellers, etc, etc. I'm sure SacCyclone will suggest others.
There's also RHDJapan for the really nice JDM stuff direct from Japan, especially if you are into Kameari goodies. That's where I got my car's Fujitsubo twin system from. The Yen rate is not as favourable as it was a year or two ago though and will get worse.
The supply situation in the UK is precarious - as shown by Mike being ill. Once he finally retires (he's not allowed!) we are screwed if needing something quickly.