Here's a sweater fleece jacket from LLBean: Sweater Fleece Jacket for Dogs | Free Shipping at L.L.Bean and one from Ruffwear: Fernie? Fleece Dog Jacket | Ruffwear Performance Dog Gear
Do you definitely want a sweater or would you consider a jacket? RuffWear and RCPets make wonderful jackets/coats. Bean also has jackets (can't vouch for them but we have the RW and RCP ones).
Once you get the sizing right per the company website, you might be able to find the products on Amazon for cheaper.