Lost a spoke and need replacement

Hi,
As the title says I've lost a spoke and would like to replace it, it's on the rear wheel of my wr250r.
I had a quote for a full set for £45 but would rather just replace the missing one.
Can I use one off a wr or yz 250 etc as the wr250r is very rare in the uk.
Any help would be appreciated.
Cheers
Dayne