Have you considered just buying one of the least-expensive models at your favorite big-box retailer? They are all certified, regardless of price, and a one-week rental of the seat is about the same amount as the price (~$50).

Here's an example:

http://www.target.com/gp/detail.html/601-8897537-9111368?ASIN=B000PBZJHI&AFID=Froogle&LNM=B000PBZJH I|Cosco_Scenera_Convertible_Car_Seat_Transatlantic &ci_src=14110944&ci_sku=B000PBZJHI&ref=tgt_adv_XSG 10001