u can go to any traffic dept located in:
1-Madinat Khalifa (main)
2-Airport Area (on E ring Rd)
3-Almamoura (near the fish market)
4-Wakra Area (on Main Wakra Rd)

working hours for 1, 2, 3 are from 0600 till 1900 hts.
for no. 4 from 0600 to 1300 hrs.

Note. Do not go at starting or ending times