A quick Wikipedia search yields 'Acorn squash is also called pepper squash, or Des Moines squash.' With regards finding some to buy . . . perhaps a phone call to Waitrose might yield something (ok, it's a long shot. An email to their HQ might be more sensible). Also, there are many 'exotic fruit and veg' shops in West Ealing, might be worth visiting them? And/or Southall.
Tony Heath ● 1958d