Mrs. Allister makes an appearance in the episode "The Pothole."  In Elaine's attempt to acquire a "supreme flounder" from China Panda, she discovers they will not deliver below 86th street [street itself is boundary].  Elaine offers to cross the street, but when she does they refuse to give her the food because "she does not live in the building."

Elaine finds an empty janitors closet in the building across the street, and puts masking tape on the door citing the room as "1Q."  She starts to have food delivered to the room... enter Mrs. Allister.

Mrs. Allister knocks on the door and notifies elaine (time #1):

"Excuse me what are you doing in there..."
"You were hangin around in there lazin on the job when you should have been downstairs in the basement cleanin out those old carpets and scrapwood."

With Mrs. Allister now hot on Elaine's case, she [Allister] becomes upset during their second interaction:

"Yes Coming Mrs. Allister" [Elaine]
"I told you yesterday to haul that trash out of the basement.  Some of the children have been playing near it and putting it in their mouth.  Get that stuff out of here today or you'll be out of here."

Physical Description:
Heavy Set African American Woman, noticeable gap in teeth, non-New York accent.  Has small dredlocks roughly 4.5" in length.