How or where do i get dynamite?

You can buy dynamite from a black market merchant for $500, or find it in various locations including a few safehouses.

To find the dynamite-selling black market merchants on your map, look for an icon that looks like a stick of dynamite slanted from lower left to upper right like this slash: /

The one that slants the other way is a merchant who sells bombs.

All black market merchants appear as a diamond shape in the mini-map in the corner of your screen.