You could try SRS
Signs in Winnipeg.... I know they manufacture in-house but I have no idea how they compare in terms of pricing.
There is also Direct
Sign Wholesale in the US who I've had quote a few projects but nothing that turned into a job.
If you do find a reliable source it would be good to know...I usually turn away channel letter jobs because it's too much work to find a reliable supplier that can quote in a timely fashion.