As of December 2015, all nine seasons of "Foyle's War" are available for purchase on Amazon Instant Video. Episodes are available for streaming for $7.99 in Standard Definition, with full seasons available for $15.99 in Standard Definition.
Amazon Instant Video is the only service which offers all the episodes of "Foyle's War." The first two seasons of the show are available on Hulu with a free subscription trial, but there is no information as to when other seasons might be added for streaming. "Foyle's War" was previously available for streaming on Netflix, but it was removed, along with many other BBC shows, in early 2015.