Chaetomorpha is what's recommended usually since it does not go white and melt like caulerpa through sporulation. In an ultra low nutrient tank in which vodka or other carbon (vinegar, sugar, etc.) is added, caulerpa racemosa will survive while chaetomorpha will go brittle and disintegrate. If you are not dosing carbon, I recommend that you use Chaetomorpha.
As for where to get it, CRA should have some or local hobbyists would be another good source.