aphids on lupins

where can i buy something to kill or help treat the aphids attacking my lupins

Doug says the easiest way to kill aphids is to knock them off the plant with a jet of water. Once on the ground, most of them will be eaten by larger beetles. If you're convinced you absolutely need to kill them, let me suggest regular sprayings of insecticidal soap. As to where you get that - let me suggest a local garden center (given you're in the U.K)