Amazon.com: Summer Metal Banister & Stair Safety Pet and Baby Gate, 31"-46" Wide, 32.5" Tall, Install Banister to Banister or Wall, or Wall to Wall in Doorway or Stairway, Banister and Hardware
Amazon.com: Summer Banister to Banister Universal Adapter Mounting Kit for Pet and Baby Gates, For Round or Square Banisters, Fits Most Hardware and Pressure Mount Gates up to 37" Tall, Gate Sold
Amazon.com: Summer Wood Banister & Stair Safety Pet and Baby Gate, 32"-48" Wide, 33" Tall, Install Banister to Banister or Wall, or Wall to Wall in Doorway or Stairway, Banister and Hardware
How to Install Baby Gates on Stairway Railing Banisters Without Drilling the Post - YouTube
Summer Infant 48-in x 33-in Hardware Mounted Oak Wood Safety Gate in the Child Safety Gates department at Lowes.com