Just ask a writer whose work you like and/or is similar in some way to yours whether they want to trade beta-reading.

Seriously, I've done this in several different fandoms with great success and met some of my best fandom buddies. Most people are pleased & flattered just to be asked. Also, if you state that it's a tryout with no strings attached, that gives both of you an escape hatch if it doesn't work out for some reason.