Amayama are excellent for much cheaper genuine parts but there is often a bit of a delay (usually sourced from Japan or UAE). For secondhand stuff there is a specialist 4x4 breakers called something like Thornton 4x4 breakers, as well as other scrappies on ebay who advertise 'Colorado breaking'.
There are also suppliers like Milner, KS International and Japanese 4x4 Spares that I have used in the past.
I dunno about specific brands of aftermarket parts? How would any of us really know (a) whether the brand is any good or (b) whether it's a genuine part from that brand (there's so much fakery these days)? All you can really do is trust that the parts dealer you buy from has an incentive not to sell rubbish parts that generate complaints, and to ensure their parts are genuine. (And steer clear of bargain-priced unbranded or Chinese parts on Amazon and Ebay unless you are willing to take a gamble.)
Example: Febest are a well-known (German-based I think?) supplier of aftermarket parts who are, as far as a trawl of the internet can tell, reliable with a good reputation. I've used parts of theirs that seemed fine and top quality. But I bought a Febest rubber hose from the air cleaner to manifold (the one that eventually splits for us all) that lasted less than 6 months. A fluke? A fake? A fair reflection of the overall quality of their parts, or a specific weakness? I don't know.
On the other hand I've recently fitted lower ball joints - an utterly crucial part, as we all know - that Roughtrax sell. They're a brand I've never heard of, but I am trusting that Roughtrax won't sell me rubbish that could be potentially lethal if they fail.
Just my musings. It might be a different matter if you work in the trade, but I don't.
