You could declare your main form's instance as a public shared variable: if your project runs the main form, you'll need to create global a main() routine for startup, i.e.
Public Shared ifrmSomeForm As frmSomeForm
Public Sub Main()
ifrmSomeForm = New frmSomeForm ' We create the instance here as opposed to in the variable declaration as that could lead to multithreading problems
ifrmSomeForm.Show()
' Insert application shutdown code here
End Sub
Then wherever you are, the main instance of the form frmSomeForm can be accessed via ifrmSomeForm without worrying about race conditions etc.