Expanders to widen slot in window

Iooking for expanders that will widen the window slot so I
That the balances can go through the slot easily.
I have Simontion Windows and I need
To replace the balances of a couple of
My windows. The balances have plastic parts
And have gone brittle over time.
I tried replacing the balances of one window and
Was a challenge to put in the new ones without
A plastic extenders to pry open the slots
A bit.
Do you sell extenders or any tool that can
Open the slots to make it easier for the New
Balances to get in through the slot.

The tool i am looking for is called
Jamb spreaders and they are made of plastic.

do you carry them and how do i order them.

thanks!