Is it an older model Britax or the newer style? I'd look on eBay, you can usually find covers there (like when a person is in an accident and the seat is unusable, they may sell the cover to get some of the cost back). Make sure you check the date of manufacture on the seat you have-- Britaxes expire after 6 years.
Mom to J (10), L (4), and baby #3 arriving in July of 2015
Re: s/o car seats:
I don't know what you have, but here is a cover for the roundabout:
https://www.amazon.com/Britax-Roundabout-Cover-Set-Davenport/dp/B00422M5KK/ref=sr_1_5?ie=UTF8&qid=1309370367&sr=8-5
and one for the marathon:
https://www.amazon.com/Britax-Marathon-Convertible-Seat-Cover/dp/B001JPGZZO/ref=sr_1_5?s=baby-products&ie=UTF8&qid=1309370424&sr=1-5
They have different colors/patterns, too.