You don't need to be an 'investor' to invest in Singletrack: 6 days left: 95% of target - Find out more
Today? Long shot I know!!
Halfords sell the oil. Eg https://www.halfords.com/motoring/motorcycling/motorcycle-oils/valvoline-synpower-fork-oil-5w--1-litre Might have other brands
Should be able to click and collect to check stock
Re use the old washer
It’s in the abys of the garage floor
As a temporary stop gap you could use rubber washers if you still have the little cup washer the crush washer sits in
15wt for lowers. 5wt for the damper.
I'm sure the Halfords stuff will work fine, might not be that similar in viscosity to Rockshox 5wt though, so might change the damping characteristics a bit.
If you have a motorcycle parts place in town that's open, then you can use the chart further down here to find out something as similar as possible.
https://www.peterverdone.com/wiki/index.php?title=Suspension_Fluid