Steve......best place to try is also a bit tricky to find, but they'll be able to help you out....they're open until 8 tonight.
Go to the Place De La Liberte...it's the one with a big low blue roundabout and fountain that has LEDs round it, and a Cepsa Petrol station. (Just near the corner of the old city walls and the one that Ave Mohammed 5th bisects)
Take the Rue Echchouada...it's on the corner of the petrol station.....go along a hundred metres or so and on the right hand side you'll go pastthe Arab bank and a load of shops...there's a cashpoint there and some boutiques and stuff.
At the end of that row of shops, there's the entrance to an underground car park- it's a really steep concrete ramp that goes right down two or three levels then curves under the shopping complex to the right......follow that down and round past the security guard (he won't say anything, he'll be all spliffed up at this time of day

) and just around the corner you'll find a well hidden bike shop- it's about the only place in Mara that does big bikes.
I was in there a fortnight ago and they had a load of tyres- a lot of second hand ones but some new, and they do have some decent kit in there (inc a KTM 990 Adv for sale but that's by the by) They also had inner tubes and all sorts of stuff in.
The lads who work there are very good and the guy behind the counter speaks a bit of English but he and the owner mostly speak French- they'll sort you out though if they don't have a tyre in- they're not rip-off merchants either.
Good luck
Bill
(PS Tell them Bill from Moto Morocco sent you

)
EDIT
PS If you have any probs finding it I'm on +44 779595 4449 tonight and can talk you in to it.
DOUBLE EDIT....By Bike shop, I mean it's a proper big-bike garage, not some crap boutique.....take yer buff 'cos it's smoky as fekk down there....and follow your nose, it looks wrong until you turn that corner at the bottom of the ramp and then find it
