These are the two best bass luthiers in Prague:
Miroslav Slustik, ph. (+420) 222 583 037
Jeseniova 125
Petr Zdrazil,
www.petrzdrazil.com
Usually they have some instruments for sale. You can also visit a shop with many basses of various quality: Atelier Paganini, Spanelska str. (near Vaclavske namesti - the very centre of town)
Good luck, Jaromir