Why do you think it's not available? Did it say it was out of stock when you tried to add it to your cart? I've heard a couple of people think they weren't available anymore just because they couldn't find it when Tesla rearranged their online store pages a little. Go to "Shop", "Charging", "Model S", "Adapters". There are the Gen 2 adapters near the top, but lower down are the Gen 1 adapters, and the 10-30 is still listed there as one of the ones you can buy. So buy it.
Model S/X Gen 1 NEMA Adapters
But if it truly isn't available anymore for some reason...I would start calling around service centers to see if you can get one. OK, now seriously, I have a few cords from EVSEAdapters, and they make great stuff, and are a really well-respected source for Tesla adapters, and that is exactly what I would order if I couldn't get the official one from Tesla. I was very happy to see they are now making Gen 1 and Gen 2 adapters with the resistors inside to properly set the current to get people away from all this stuff of trying to convert everything to a 14-50 outlet.