Here are some links with pictures. Note that they only have one notch on top as opposed to the two or three that are on a normal utility knife. Otherwise, they are the same as a regular utility knife. You will need a special utility knife designed for them or just modify an old knife (by cutting of the extra protrusions that the notches fit on) like I did.
http://www.tools4flooring.com/asr-personna-carpet-trimmer-blades-p-211.html
http://www.toolbarn.com/product/personna/61-0021/
I see someone has them listed on ebay but haven't recieved a response yet. I sent the seller a note to comfirn this is actually a carpet trimmer blade. They are shown on a Personna display guide in our local Lowes (in the flooring section), but I haven't seen them in stock so far.
The best way to get them is through a carpet installer. They will know what you are talking about. I would guess they would sell you some for a very reasonable price.
In looking at the assortment of blades at Lowes I believe some of the other types may be beveled on one side too.
Ned