There are a Couple of Shops you can try :

Sultan tires : right beside the KFC on Salwa Road,

They will even exchange ur rims and probably give u a discount on the new 1's ...

There's also a shop inside of Teyseer Service station on Salwa road the place has all kinds of sizes and brands of rims there !!!

And another one at the Wakra round-a-bout with he pearl thingy...

He too exchanges old wheels for new 1's..

Hope this helps.