I'm not an expert about the correct fabrics for tutus but there is a treasure trove of a shop, Joels in Church St, off Edgware Rd, (north of Bakerloo line station where the Marylebone flyover crosses Edgware Rd. In Church St (which is a street market - quite rough), Joels is on the south side of the street, diagonally opposite Tesco.
They have remaindered rolls of all sorts - but only ever one roll of everything. Some stuff is cheap, some are couture fabrics and though still not cheap are much more realistic.
You really have to delve - nothing is displayed, lots of rolls piled up on shelves. Easy to get swamped, but well worth a delve.
If you are intereseted I'll find the phone number. You hav eto be quite tough to get the staff to be helpful.
It's where my wife bought silk for her wedding dress.
Can't rememember if they do trimmings, beads, braids etc. Here best bet is good old John Lewis on Oxford St and there is a shop in Marylebone High St, sorry don't know name, on corner of a side street, on right as you head north, lower down than Waitrose (on other side).
Others will I'm sure have there favourites too.