I think a Generic Honeywell key would work, maybe this: 17003 Key Set | Pull Station Parts | Manual Initiating Devices | Voice & Notification | Pull Station Parts | Honeywell Building Solutions
According to the datasheet, it should take the 17051 key.
While I’d normally recommend Beach’s Lock, which is a great place to find fire alarm keys, looks like they don’t sell the key you’re looking for. This ebay listing does, but it’s a little steep.
In the future, I totally recommend Beach’s Lock for all your fire alarm keys. They even sell a set of 11 most commonly used keys for like $30.
I agree with Sly, a 17003 key. I have its brother the BG-12L and it takes the 17003. That lock doesn’t look double bitted which would be a 2126 or 17051. (same key)