How can I extend my visa while in Chiang Mai before it expires?

My Visa for 30 days expires March 29th....

I want to try to extend my Visa.

That would expire by the end of April.....

My return flight home to Canada is May 15th.....

thinking about changing the date.......... due to Visa.

I am planning on being in Chiang Mai March 18th to March 28th

Where is the best place to get an Extension on my Visa.....

I am staying in Old Town.

Do not drive.....

Any help would be appreciated.