I question that Alfa Parts Exchange is the "equivalent" of Classic Alfa or Alfaholics. APE is an Alfa dismantler - a one marque wrecking yard. Neither of those two UK suppliers do a big business in used parts...Also, Alfa Parts Exchange.
Absolutely! My brief list wasn't meant to be all-inclusive. Off the top of my head, there is also Eriminas, Alfissimo, ....I would add DiFatta Brothers of Baltimore to the list for quality OEM type parts and Performatek for high performance suspension and brake parts.