Reaper has made several imps, they are in some of the Familiars packs.
#02399 Familiars Pack II (1 imp plus 5 other familiars)
#02756 Familiars Pack IV (1 imp plus 7 other familiars)
They also did a couple of sets of Blood Imps.
#02197 Blood Imps (3 different imps)
#02427 Blood Imps (3 different imps)
Then WOTC had an imp (#43/60) in the Angelfire set of their prepainted minis.