I buy most of my ingredients for my DIY soylent at www.bulkpowders.co.uk
They are quick, efficient and the powders are very good quality. Here is a discount code - get £5 off your first order.
The products I buy from them regularly are:
Pure Whey protein
Psyllium Husks Powder
Soya Protein Isolate 90%
Ultra Fine Scottish Oats
They also do really good cheap blender bottles. storage tubs and scoops.