The white sand you found in Guadeloupe is the same white sand that can be found in more than 15 Caribbean Islands such as Tortola, St Lucia, St Vincent, Grenada, Trinidad, etc. This white sand comes from Guyana, not from the beaches, but from local quarries developed and approved by environmentalists to supply the new demand for sand in the Caribbean. With the new environmental restrictions regarding the sand mining in the Islands, Guyana created a special sand export business as the chief solution for the recent sand crisis the Islands are facing.
Martinique is a country in the line to start importing white sand. I don’t have the contact information in Guyana to give you, however, you can make a research. There is only one big company in charge of exporting the white sand from Guyana to the Caribbean.