Do you want an actual vintage stem or do you want a cheaper re-pop? There's a couple of options here.
If you want to find a authentic vintage stem:
www.bmxmuseum.com It might help putting "Freestyle" as a key word as that was a freestyle modification for rotors.
Your in the US? You can buy modern re-make versions here.
Stems: Quill - BMXGuru.com
Porkchop BMX Quill Stems Old School BMX Bicycle Products
Shafted BMX Stems Archives - Americancycle.com Order BMX Bikes and BMX Parts
I'm not sure how handy you are but BITD it wasn't un common for people to drill and tap the hole themselves, and add the adjuster bolt from some old brakes. So you can spend $16 on a new one and try drilling it yourself. Or expect to spend over $100 on a vintage Suntour in good shape.
Btw the one in your picture is a combination of two different suntour stems.
See the stem on the left? The hole is there for the allen key to adjust the stem bolt. They took the cap off that and added it to the stem on the right.