You can get them from just about any labware supplier, at least you can where I am (and that's saying something!). Usually they're really cheap (in comparison to porcelain and glass) and often the buchner's will have the rather snazzy feature that you can unscrew the top (with the disc) from the bottom 'funnel' part, for better cleaning (sometimes these one's cost a bit more, but I personally think it's worth it in most cases).