bialetti replacement rings
The houseware store on the main foor of the St. Lawrence Market (right in the middle, basically) also sells the rings in many sizes. A good idea is to take your old one out first and bring it along, or at least put it on a scrap of paper and trace it out, so you can compare the sizes at the store. They keep them behind the cash and they cost about a $1 each, if I recall.
I did have some problems getting rings of the smallest size Bialetti, though; I had originally bought it in the Corsa Italia neighbourhood but few other places sell that particular size of maker for some reason, and therefore don't sell the rings either.
Colangelo sells them. Midland and the 401 (even though they don't intersect.)
Probably Pusateri's sells them as well...
Best to give either of them a call first as they may not carry all sizes. May as well replace the filter at the same time.
And there is always these places...