Hi Happy,
I agree with space_cadet this is not a good idea.
Apart from there being a risk of getting an infection from a second hand lens, any second hand lens is extremely unlikely to fit so if you did get it in you'd be desperate to get it out again as it would probably be extremely uncomfortable.
Why do you want to practice putting them in your eyes?
If you are going to be prescribed scleral lenses you will be taught how to use them before being sent home with the lenses. Opticians need to be sure that you can safely insert and remove your contact lenses before letting you take them home.
You might find it useful to look at some videos produced by Moorfields which demonstrate how to care for, insert and remove various different types of lenses, including sclerals.
https://www.moorfields.nhs.uk/content/c ... l-and-careLia