Duplicate the products Tutorial: What are Table Occurrences?. (Select it and click the button with two green plus signs). You can double click this TO to get a dialog where you can rename the new copy to something better than "Products 2". Link this new TO to Purchase Orders by the supplier fields. Now you have the typical two table relationship used in what I call a "basic conditional value list".
Set up a "use values from field" value list to list your values from this new table occurrence and be sure to select the "include only related values starting from Purchase Orders." option.
For more on Conditional Value Lists, you can check out this demo file with 10 different ways to set up a CVL:
I apologise for the late response. That is the perfect answer. I had a look at your linked file too. It is very helpful and I recommend anyone who is considering this approach to have a look at it. I guess in my mind I was thinking more of a hierarchical value list as you described it, but the plain old conditional value list works just perfect for what I need.
Once again thank you for all the assistance you have provided.
What fabulous work!! I found this "adventure" invaluable in helping me understand conditional value lists. The examples are excellent. Well done!!