I think someone on here posted a picture on here of a homemade filter awhile back. If i remember correctly it looked similar to this but much smaller. Sounds like this is different than what you have though. http://www.englishteastore.com/glass-teapot-manon.html
I imagine its gonna be pretty hard to find one like you already have in a store or online but maybe someone like Scott could source something similar for you.