When I learned I couldn't get 455 anymore, I ordered some of these from Comet:
https://www.cometpinball.com/products/455-blinking-bulbs
From their recommendation, I'm using both fast and slow bulbs in the same machine. This does make the blinking seem more random if you alternate them. The warm white color isn't a yellow as the incandescent bulbs. But these might be the best option available right now.