Konstanz - Fabric
Hematex, Neugasse 7 (small & worth a rummage)
Minke Stoffe, Bodanplatz 3 (gem of a fabric shop - only open until 2.30 on Sat)
Karstadt, - top floor - generic insipid stuff but ok habby dept
For fabric online try
www.stoffekontor.de - great selection & prices
Muller - top floor Marktplatz kids art stuff
Great Müller toyshop under Depot/Interior (Marktplatz) - entrance down the side