Promark 20 Inch 100 LB Capacity Full Extension Soft / Self Close Ball Bearing Side Mount Drawer Slides - 10 Pair Pack
https://smile.amazon.com/dp/B00B854SMC/ref=pd_luc_rh_sbs_02_01_t_img_lh?_encoding=UTF8&psc=1 $9.40 per pair for 20"
Available in both 20" and 24" lengths.
Some people think it is best to get Fulterton slides or Blums bottom mount Blumotion slides. Knape&Vogt are also ok. All these named brands are unnecessarily expensive.
26" KV Soft Closing Drawer Guide (pair)
"Recommend ordering the drawer box "DEPTH" the same as the guide length. For example, if you have a drawer box at 10" wide, by 4" tall, by 18" deep, then you should order a 18" long drawer guide."