For making any Business Object accessible in any webpage, i inherit the webpage from ApplicationBasePage but i cant do the same with a User Control, so how can i make any business object available to my User Controls ?

What we always do is create a property and then set that property in the Load or the property sheet (depending on how you design it) of the page that uses the control.  This way the control is encapsulated, as it should be.

For example:

Public Property MyCustomers As CustomersBO
...
End Property

This way your control can use the BO that comes from the ApplicationBasePage by simply setting the property when the page is loaded.  Then your control will reference the property instead whenever it needs to access the BO inside of the control.