Here are a few ideas:
1) Your providing bank might be able to review your vendor master file and identify the level of acceptance among your suppliers.
2) You could conduct a simple survey with your vendors regarding their acceptance of payment via p-card.
3) You may want simply to notify them that you are changing your procurement practice and will now be purchasing from them using a p-card, and see how many accept and how many require follow up. Your success may depend on the leverage you have with your vendors.
Here are a few tools, available to you in TAPN's AP Tools Suite, that you might be able to modify to serve your purposes:
You also may want to post your question in TAPN's Forums to seek ideas from other members about whether and how they determined level of p-card acceptance among their vendors.