Getting UL certification for neon signs
I'm afraid you misunderstand UL certification. "You" cannot get certified. Only your product can. Furthermore, each different product would need to be certified separately. This would involve providing at least 3 samples for destructive testing, money in the range of $10K+, and a wait in the range of a year.

For a maker of custom neon signs, I can't imagine that this would be a good idea.

If you like, you can get a copy of UL48 (electric signs, including neon) and see what standards would be applied to your signs, and use this as a guide in building them.

But take Adam Lawrence's advice, and get your information from UL, rather than some stranger on the internet.