The secret Blueprint Shop vendor randomly appears in specific locations. Look for a D.O.D. Arms dead end wall - a different special vendor appears in each section of the Tower Of Barbs. Search the indicated locations on floors 1F-10F to find the Blueprint Shop vendor. The vendor's location randomly changes every time you leave/enter the Tower Of Barbs from the Waiting Room. Note: To fully explore 1F-10F, you need to defeat the first Don on 10F: OMENO-SOTO or JIN-DIE on 6F: MOKA-MOGAME.
3F: TAGAHARA - It is on the main route, located at a dead end, near the elevator.
5F: DENDEN-CHOFU - It is accessed through the blue escalator exit from 4F: TAGAHARA.
3F: METAKOJIYA - From 1F, travel to 2F: WANOKI (orange exit in 1F), and using the purple exit.
9F: YONEGUICHI - From 6F: MOKA-MAGOME, use the blue escalator to reach 7F: DANEDA. Use the purple exit to each 8F: META-KOYA. Then, use the blue exit.
If you have an Express Pass, 1-Day Limited Express Pass, or lots of Kill Coins, the easiest place to reach is 3F: TAGAHARA. Just take the elevator up; if the vendor does not spawn, return to the Waiting Room and try again.