Harry
Just google "windscreen washer tank" or "universal windscreen washer tank" and you will find plenty of hits at various prices. Car scrappy or breaker might get you better quality.
Cant help you with a bypass & splitter system. Just something I've used on a piece of equipment and found rather more controllable than a simple tap.
Roberts suggestion of varying pump power input works quite well bbut these small pumps tend to have a rather non linear variation in amount of liquid puped against power supplied. My experience is OK control over a certain range, down to maybe half, then it all gets iffy. really want something thats about right in the first place rather than hoping to considerably reduce the output from "I've got this one but its way too powerful" pump.
Clive