Wagner and Gemini have both discontinued their slotted changeable copy letter product lines. Third party suppliers may still have some to sell, but obviously the stock supply is limited and diminishing. Some companies are custom making slotted letters. I'm going to a sign industry regional conference the end of this week and hope to find out more about this situation at the trade show.
I think slotted letters look more classy than the flat Zip Change/Pronto style letters, but they require more sign face space and are more expensive. The cost of LED-based variable message displays has fallen enough that many customers opt for those types of signs rather than the old "analog" signs that have to be changed manually.