Messerschmitt Parts

It is possible to buy much Schmitt stuff without being a member of anything. I have done it for years. At the moment I am tied into the MEC but if they decide to not stock spares again I will be back to total independence. Yet I have moved at least 4 schmitts a year on.
So the first part of your question is what you mean by best? Most people mean easiest. If so you have your answers, though you can include Van Rooy in Holland. However to me best means the best part I can get at the best price. Sourcing privately can save some £1500 per car restored! You will not always manage that buying from the obvious sources. Either the price is above the background level or the part is not as good a replica of what you want to fit and other options are better. There is nothing so frustrating as spending a small fortune, schmitt bits are not cheap, restoring a car only to find it is 50% incorrect and substandard and held to ridicule at a Schmitt gathering. Thus you need to know how accurate you want to be in your restoration to answer what is the best source. For instance no one supplies a totally accurate reproduction floorpan. So I can spot all those with replacement floors. This might not matter to you. It does not to me but it does if someone is claiming their car is concours when the floor is not correct! (This is not a complaint. The tooling needed to produce a totally accurate floorpan would cost a fortune and best compromise is available. However if you want best it is not available for this part. So original rust free cars claim a premium price as they should)

Hubcaps, Wheel trims?, are available off eBay and possibly Andy Carter, unless you have an early car, those are not available. Hubcaps would have to be second hand and again two types.

Ignition barrel, not sure you can buy those. There were Lucas ones about which do the job. Second hand switch complete or a serviceable lock of the same pattern would donate a working barrel. Keys can be cut.

Front light, which one?
Headlight, two sorts - replica late one available. I repair the reflectors, technically illegal but I have never failed an MOT with my way of doing it. Hella glass can be bought in various places cheap. Bulb holders second hand, Bulbs available anywhere good, Rims - a good club part in ali but not as good a fit as an original with hanging rear edge, so I look for originals new or restorable. If you just want a light then if you have a rim you can get a replacement for about £15 complete.
Sidelight, the club sell plastic replicas of the later Hella unit, very good but not original. These can be bought direct cheaper. You still need a rubber that fits, differing sources again. If you have an early car, whoops, they start at £100 a side rising to £400, this includes reproduced ones!

Dome. Get hold of Heiko and arrange a short holiday to Germany when the next one comes available so as to make the most of collecting it. It is a lot cheaper by the time you have bought the rest of the parts you want cheaper from the German sources! Better still take in a German autojumble.

So nothing there that needs joining a club for unless you want to dial a part. Without research you still will not always get the best item or deal available. Up to you as to what is best in that lot and joining a club is still a good idea despite the spares issue.