At the date of this writing, until the Glock parts suppliers get stock, the surest way is to get a Glock Armorer to order the Gen5 extended slide stop directly from Glock, which will typically take ~3+ weeks before receipt of the part.
Further Info: Glock sells the part for $8, but has a minimum $5 shipping charge with sales tax for the shipped-to-location charged on the total $13.
Glock could also be out-of-stock for a part at the time of order, too, which would delay receipt should an Armorer place an order.