Just been to the Gem And Rock Show in London and was looking around for Moldavite but none of the exhibitors had them so still on the hunt.

From my research it must come from the Check Republic ..more research shows various sites selling them and prices vary as some are very expencive .

Moldavite has been found in four countries. The main deposits of Moldavite exist in the Czech Republic where more than 99% of the Moldavite is found. The other countries include Germany, northern Austria and southwestern Poland. However, these Moldavites are rare collector items and you won’t find them in any shops.

Typically speaking, all of your Moldavite should come from one country only which is the Czech Republic. The below map shows the Ries impact crater and the areas Moldavite landed in.

If you the description says the Moldavite is mined from any other location than the Czech Republic or is described as manufactured then it is fake.

The areas where Moldavite is found

Now for wearing as a pendant how many grams would be suitable enough to show results in rapid transformation.. you dont want to wear a big stone as it would be heavy and expencive and then again you dont want to wear a small stone as it would not seem to activate rapid transformation so a middle size would be ideal. 
Here are some sites claiming to have genuine ones :

https://www.innervisioncrystals.net/collections/moldavite-1-to-5-gram-stones

https://myfaireden.com/blog/where-to-buy-real-moldavite-online/

https://silverskysmoldavites.com