Depends on how much you want to spend.
Fusion and TR-Enabling (Remsport) sell blank slides. Same with Caspian.
I think RIA actually sells a 9mm bull barrel slide with LPA sights, like the .40 one your gun came with. Check Hooper Gun Works out, they sell a lot on their site and on Ebay.
Or find a pre-cut slide at Shooter's Connection that you like.