Jan De grote Kleinvakman
They have just about every notion that you could imagine, and some that didn't even occur to me before seeing them there (everything from crochet flowers to safety eyes). They have a huge wall of trims (and then more in bins). Unfortunately my picture of the whole wall turned out blurry, but below is a peek at one tiny bit... just picture this extending floor to ceiling for a long, long ways. One type of trim I'd never seen before was bias tape that was polka dotted and then edged with little lacy scallops. It came in about 6 different colours and was so pretty!
And taking up nearly as much space as the trims were buttons! All the trims and buttons were nicely sorted by colour or style too, which makes it easy to scan for what you like.
For fabric, head back out of this dimly lit cavern of goodies, and into the market. There's one booth that sells "craft pieces" that are pre-cut lengths of cotton. There's a huge variety in style from strawberries and flowers to Japanese block-print batiks to hilariously touristy stuff with blue windmills and tulips. I couldn't resist this map patterned piece today... and I don't even have a sewing machine here yet! Incorrigible!
Global Map fabric
If you want to find this booth particularly, it's nearer the Ferdinand Bolstraat end on the South side, and he's usually only there on Friday and Saturday. But there's lots of other stalls selling all sorts of fabric off the bolt too.
Rolls of batik fabric
Update: There's a website for the stall that sells the pre-cut lengths: Hobbylapjes.