Are you Canadian? If you are, it would be easy for you to get it from there. Why would you need a police clearance certificate? Are you going to work here? If you are then you won't need one, unless you have worked here before and needs to transfer your sponsorship from another employer to the new one. If this is the case, you definitely need a police clearance certificate from Canada (if you are holding a Canadian passport). You have to have it authenticated at the Ministry of Foreign Affairs in Ottawa, have it stamped at the nearest Qatar embassy in Canada or it can also be done in Canada embassy in UAE.

You can also get a police clearance by applying online at Truecheck police. Its an online service that can help you get a police clearance from Canada. They are fast and efficient too.

If you need to have it authenticated without trouble, there is an authentication company that will do everything for you. Fast and safe for a reasonable fee.

Let me know if you are interested and i will give you all the details. My answer is turning into a novel already :)