There are occasionally parts that the usual suspects might not have that can be found in a junkyard or off somebody's parts car, but other than that, I would presume that people like Banks, r.d., Dave Bean, et al. know the proper equivalent parts on other cars and would would use supplies of them if Lotus had supply issues with those parts. In fact, I might expect them to stock the equivalent parts before the 'official' Lotus parts if they are cheaper and truly equivalent. Those X-reference lists are not secret and I imagine they've been through shortages of parts from Lotus even when the Europa wasn't so old and may have had to be creative.
If you pay a little more from the usual suspects, you could consider it a finder's fee, or a research fee, or even a loyalty fee. You might be able to save a $ here or there by buying the equivalent part at Moss Motors, but I doubt you'd save that much and I think it does hurt the lower volume Lotus parts suppliers since they don't have as much room to maneuver as the bigger BL parts outlets like Moss or Victoria British. You can get a Triumph part from lots of places, but you can't get a Lotus part at that many.
I would also suggest that Dave Bean might be a good place to start your parts search. Dave recently lost ALL of his personal possessions in a fire. Certainly a lot of that will be covered by insurance, but I'm willing to bet that a lot will not. Dave has a lot of interesting parts and experience that most people don't have (in some ways similar to Richard at Banks but certainly not the depth of experience with Europas that Richard has). People like Dave, Richard, and Ray have experience with Europas that you'll never find anywhere else. Sometimes, the part is just the start of the conversation.