You might check with some of the English pubs in LA or OC. For instance the Cat & Fiddle makes their own and they might do one for you. If not then call some of the other well known ones like Ye Olde Kings Head in Santa Monica and see if they know where you might buy one.
Is this what you were looking for:
"TRIFLES - No British party is complete without a Sherry trifle. Just bring us your bowl and we will take care of the rest."
Small - $15.00
Medium - $20.00
Large - $25.00
If yes, then The Tudor House in Santa Monica can help you.
Click on the "Catering List" (in the middle) then scroll down to the very bottom: