I recently reinstalled my 90 degree adapter and had some leaking issues. I called the local dealerships, they said no stock anywhere on the gasket kits. They "estimated" up to 6 months or longer to get one. Your best bet is mancini or summit. They sell them.
What I ended up doing was using the felpro 70522 on the block (make sure it clean on the block and there is no gasket material left there). I used lots of permatex gasket dressing on it. Let it get nice and sticky. It helps hold the gasket in place and seal it against the block. Then I used a copper crush washer under the bolt head. And reused the gasket between the bolt and block. I also held the adapter against the block and spun it around. You'll notice it tends to fit better at a certain angle. Generally where the filter is aimed slightly down towards the torsion bar. I used lots and lots of torque on the bolt and haven't had a problem since.