TurntableNeedles.com is excellent and trustworthy. I've made several purchases from them over the past few years; all have gone smoothly, and staff were helpful when I had questions. I typically use TurntableNeedles if what I want/need isn't available from V-M Audio Enthusiasts.
I've bought new cartridges and new OEM styli from Needle Doctor (and had good experiences doing so) but I've never bought aftermarket replacement styli from them.
As far as Stanton styli are concerned: your best bet is to buy directly from JICO if you can afford to. Of all the aftermarket styli manufacturers, they come the closest to replicating Stanton's suspension system, which is a huge factor in how those styli sound and perform.
They are not inexpensive but they're almost always more affordable than new old stock OEM, and you don't have to worry about stiff suspensions that have hardened from age.
the thin end of the wedge