I suppose it depends on who has the plugins you need; you've already mentioned a few good sources. Boris also is well-known.
I tend to buy direct from the creator when possible. But Toolfarm is often a little cheaper, and a perfectly respectable outlet.
I try to take advantage of sales when they happen. In December, Red Giant had a 1-day 40%-off sale, so I grabbed some extra licenses for Trapcode and Magic Bullet suites. Subscribe to all the supplier newsletters - Red Giant, Boris, Toolfarm, Zaxwerks, Gridiron etc - to keep abreast of bargains.