From reports I see on line, the cheap Chinese calcite dichroscopes are fine, so you can go with whichever suppliers seems best/cheapest. Get a return guarantee and try it out. These are simple, a piece of transparent calcite and a lens, so they should all be pretty similar if the calcite is of decent quality, and such is not difficult to find. The London dichroscope, made with crossed polarizers, is better for viewing parcels at a distance, but I don’t find any of these for sale on line. You could pretty easily make one from some polaroid sheet or two polarizing sunglass lenses, though. About $25 or so should do it for the calcite dichroscope on Ebay. There are offers for a dichroscope, a triplet loupe and a Chelsea filter for about $55, and that might be a good deal…compare prices.