Pixologic.com – A collection of alphas from the ZBrush community collected by Pixologic.
Textures.com – damage, cracks, rocks and screws.
Zbrushguides.com – hair, feathers, fur and more.
ArtStation Marketplace: Brushes & Alpha Packs – a platform for artists to sell their products, alphas included.
BadKing.com.au – environmental, tech, cracks, wall, stone and more.
Want your alphas added to this list for free? Contact me.