VB6 doesn't have a native slider control.
But you can use these additional controls to get it (Project->Components):
Microsoft Windows Common Controls 5.0 (SP2)
Microsoft Windows Common Controls 6.0 (SP6)
There are also third-party, open-source controls you can use, like:
vbAccelerator
CCRP
If you mean "splitters" (lines to help resize panes in a dialog) then those are only available third-party too (pretty sure vbAccelerator has one).