The National Gallery in Washington has just opened a show of the work of George Bellows - I hear it's wonderful and can't wait to see it! http://www.nga.gov/exhibitions/bellowsinfo.shtm
The newly re-opened Barnes Collection in Philadelphia includes many works of William Glackens and examples from some of the other Ashcan painters.
http://www.barnesfoundation.org/collections/art-collection/collection-search