Try GJ Beads or Stitchncraft - you want to look for miyuki seed beads is size 15, 11 and 8 - stitchncraft have all of these. GJ has them too.
They have online stores (google) and physical stores too. Stitchncraft are amazing and based in Dorset. Great service and I've always had fast delivery from both them and GJ beads.