I don't think you're going to find one. 1/2 is just too small for that amount of flow. Most circulation pumps this size are going to have 1" inlet and outlet...some will have 3/4".
If you're looking for an external pump, I'd recommend a Blueline 30HDX which is rated 1110 gph
and will flow about 760gph @ 4'. The 40HDX is rated 1270gph and 1020 @ 4' head. If you want a submersible, I'd go with a Mag 12 which is rated about 1100 @ 4' head.