What exactly is a 'coaster' tap? And why would it be a non-standard tap?
As 2weis advised Bunnings have a large range of washers - you should be able to match one with the washer from your tap.
If unable to get one exactly the same it may be possible to trim a near enough washer to fit - I have done that a time or two. Cheaper than a complete new tap.
I have an idea that mixer taps are sealed - somebody more familiar with plumbing can confirm. I do know that I had to replace the whole tap in our house at one time.
Anyway, unless you want an exotic tap they are reasonably cheap.
Not the mixer tap, but the tap in our Jayco Poptop, which is a very small discharge pipe. I never could get a tap washer for it - always too small. I made up my own from a sheet of insertion rubber, but that used to tear and the tap still dripped constantly. Finally, I took a washer of a roofing screw - perfect fit. Then I got some valve grinding paste and a buff and ground the valve seat down a little. No more drips - the water ones at least...
-- Edited by erad on Tuesday 2nd of February 2016 04:39:28 PM
[just as an aside, you will find HEAPS of information in Mr Google and on Youtube - they are my first ports of call when I want to know something and/or how to fix something]
Cheers - John
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2006 Discovery 3 TDV6 SE Auto - 2008 23ft Golden Eagle Hunter Some people feel the rain - the others just get wet - Bob Dylan