When I first started fly fishing I got tennis elbow from casting too. I referenced it here and one of the members recommended the Serola Gel Arc elbow brace for tennis elbow. You can find it here: http://www.serola.net/serola-gel-arc-elbow-brace
I was able to order it through Co Op Home Healthcare on MacLeod Trail S (Beside Sleep Country) and it was amazing. Using a gel, like deep cold, or Voltaren, and working it into the tendonitis area really helps too. Wear the brace while fishing, to prevent stress on the tendon, and use the gel in the morning and before bed.
It went away for me pretty quick, and haven't had to use the brace since.
*knocks on wood