You're quite tall aren't so why not look for something a bit bigger? I'm 5ft4 and just got a 17hh boy but he doesn't ride big, so it's cool.
I've found both of mine through word of mouth so you need to start putting the feelers out locally. There are a few ads up on horsemart and horsequest. You could always put a Wanted ad up yourself - be really honest about what you want and your ability, and see what happens.
Ive put a few wanted ads up and am very much having the feelers extended...
I think its because i had a nasty fall off a 16.3hh and ever since then the thought of something big like that sends me into a frenzy of worry- its def mind over matter!