Bin a few years since I lived in Shatin, but as no-one else has answered I’ll give it a shot…
You could try those bike hire places down by the concrete open sewer thats euphemistically called a river! They do sell bikes and components as well as hire them out, but there won’t be much choice.
However, you’d be better off taking the MTR into Mong Kok where there’s a few proper bike shops near Prince Edward station, down Fish Street I think.
For a FinishLine chain cleaner you may need a shop that caters more for the expats – in which case you’re looking at Flying Ball (Kowloon) or Friendly Bike Shop (Silvermine Bay).
Out of interest – what are you doing with a fleet of bikes in Shatin – would’ve thought the rental market was already well catered for by the local hire shops?