From Amazon, I bought a $15 foam Britax infant positioning insert which is certified for use with the following britax convertible car seats: roundabout 55, marathon 70, boulevard 70, boulevard 70 cs, advocate 70 cs. Note that it will NOT work with the cheaper Britax Marathon sold by Target.
This is for use with infants weighing between 5 and 11 lbs.
I realized after buying it but before using it that the Britax Boulevard seat we'd bought is too big for our little car so I'm getting a different brand of carseat.
If you need an infant insert for a Britax, I'd be happy to send it to you for free. If you want to send a few dollars my way for shipping that would be cool. But my objective is just not to have to throw it away. (Amazon said don't bother sending it back to them - not worth their while to put back in inventory!)
Send me a personal message if you want it!