Need multiple workbooks to reference a dynamic list based on those sheets

What I mean is I created a Purchase Order form in Excel with a bunch of formulas and dropdowns. Each manager will have a certain set of PO numbers they can use, or there will be a main list for everyone to pick from if I can figure that out. My issue is I have a template PO form that I would open up. I then need it to either show me the next number to use or insert it. I would then save the PO workbook as something else once I finished filling out the data. My issue is how can I get the list of POs to note what has been used already and what's next for whomever opens the new template?

I tried having the list in the same workbook. That works great, but only once. Heh. I can either have the list as a pool for everyone, or each manager has their own list on their PC if that makes it easier. I know I could just have a manual way to reference the list, but I'd like to remove human error.

Thanks for any suggestions.