I am sure you have your answer already but in case (not going to read the whole thread):
You can pay $40 at ProAc (Modern Audio, contact info on ProAc's website under world distributors)
or you can pay $4 at Madisound, just call and ask if you need help identifying the right ones.
Personally, I would definitely pay the extra $36 so the foams would have the "ProAc" stamped on them, but then I'm completely nuts.
Of course, it is not new tweeters you need. Some people think you mean foam surrounds as in the foam suspension attaching the driver to the frame, but these tweeters do not have foam surrounds. And should you ever get to this point: if you do need to replace those tweeters, do not believe people who tell you the tweeter at Madisound is the same as the one from ProAc. It is not and will not sound the same. Get them from ProAc, they have them still. If you need more info read the vast discussion about 2.5 clones. the difference between the stock and the ProAc versions are exactly why cloners could never get the sound right in the clones. Plus the same deal with the woofers. Get any replacement drivers from ProAc.
Stunningly lovely speakers, no? Would love to hear where you found them and how much, if you feel like it...
Jim