unidirectional (cardioid) electret capsules at Digikey
Search Digikey for “unidirectional” to get these results.
The Digikey catalogue page 1278 lists all the unidirectional electret capsules available from Digikey. pdf here.
One of the likeliest available candidates is the Panasonic WM-56A103, though the frequency response is only 100Hz-16kHz.
…but note that as stated by Henry Walmsley on his cheap soundfield mic site, the WMA-55A103 is the preferred model (back electret) if you can get it.
Knowles Acoustics also lists some unidirectional electrets here, but not all seem to be available from Digikey even though they are an official Knowles distributor.