The engine baffles are sold in a seperate bundle together with some resin engine bells:
http://www.greenstrawberry.cz/star-destroyer---engines-bells.php
But there are also some 3d printed ones on Shapeways:
https://www.shapeways.com/product/5EPZDLJEP/star-destroyer-zvesda-devastator-engine-deflectors
https://www.shapeways.com/product/33KBCR4EC/devastator-flaps-no-sprue
http://www.greenstrawberry.cz/star-destroyer---engines-bells.php
But there are also some 3d printed ones on Shapeways:
https://www.shapeways.com/product/5EPZDLJEP/star-destroyer-zvesda-devastator-engine-deflectors
https://www.shapeways.com/product/33KBCR4EC/devastator-flaps-no-sprue