what type of sand to use?

Silica sand it what you are looking for.

Should be fairly easy to find, as it's used for sand blasting (bone dry if stored properly and inexpensive).

Wear a breathing mask/cloth when pouring it as the dust is not something you want to inhail.

Perform the task outdoors.

Research silica sand for your application in order to judge the previous sandbox responses to your query.

I used washed sand in my speaker stands.

The best sand in the world resides on the beaches of Port St. Louis in the Maritius Islands... off the SE coast of Africa.

well... there and the Saychelles.

If money is tight though... Palma de Majorca will probably do almost as well... off the S coast of Spain.... and the night life is better.

take along some sealable freezer bags and SPF 40.

Black lava sand from Hawaii works well and goes with anything. I would caution however about using real lava despite its excellent sonic properties.