The Canon EF 300mm f/2.8L IS II and 400mm f/2.8L IS II lenses have been available at times in limited quality; however, it looks like B&H has them both available now.  Hit the links above for details.

Note that the 400mm lens is listed as “available for pre-order” which generally means that B&H expects it to be in stock and shipping out to customers within the next few days.