Where do i buy from locally?
Do all bike shops sell them?
17" but i cant fit a mountain bike wheel inner tube of 17" surely???
All motorbike (tyre) shops will sell them.
IIRC, you’re local to Watling? I don’t like them, but they will definitely have them in stock.
I think they’re standard size for the 17" spoked wheel motos. You can get “heavy duty” once though, which is worth it.
FWR sell them, but I think you need to order them. how about InMoto?
There are some Continental 4.5mm ones on ebay it’s always worth having a spare set, or at least get some decent breakdown cover. 
Mountain bike tubes will never hold enough volume for moto tyres
done sorted. £15 fitted from AC Bikes in Dartford! Top service again. Heavy duty tube fitted