I got a bag from Roland UK. They were a few pence each, no shipping charge. Drop their service dept a line via the website - I've found the guys there very helpful.
quote: 3. The dimensions of a replacement rubber grommet are: Outer diameter is ~1/2" Inner diameter is ~1/4" to 5/16" Thickness 1/4" Groove Diameter is 3/8" Groove Width is 3/64"
There are countless other threads as well, but that one has the gist of the info you are looking for.
Laurie wrote one up as well with some pics, if I remember correctly.
I've bought some from Roland here in Western Canada and they were also superb for service, but the "guides" were maybe $1 each, so they only go into the rarer pedals - the regular grommets mentioned here http://www.bossarea.com/forum/topic.asp?TOPIC_ID=4175 go into the boring/cheap pedals.