I would look at the plans to see if the ladders are referenced to a part #. If the plans show just the construction of the ladder, then it probably needs to be made using strips of wood supplied. Another hint might reveal itself if the kit came with a PARTS or Parts contents sheet - sometimes the parts content sheet might say something like 1 x 4 mm x 100mm (ladder construction)
One last thing to look at is the laser cut boards. Sometimes the ladders will be laser cut and will need to but cut out from that.
My Druid from Unicorn Models has laser cut parts on sheets of wood that is literally paper thin !!! So, make sure you look NOT in the plastic parts bin, but take a look at your laser cut boards.