I agree with Art - social media is your friend! There are quite a few Facebook WordPress groups, some for devs, some for content owners, some for beginners, etc. which may be worth pitching in. Take a look at them, check out the recent posts and the group rules and post in the most relevant.
Or even ask the same question in those groups - there are plugin developers there too who most likely have been in the same position and might know of a better way to find beta testers.
Twitter might be worthwhile too - include relevant hashtags. #wordpress is a must - there are some WordPress aggregator bots that retweet anything with a #wordpress (personally I tend to hide these bots as they tweet too much, others find them useful).
Finally, check out your local area for any WordPress meetups you can attend. There is a great monthly meetup in London which has a fantastic community spirit and feeds back into the great WordPress ecosystem, so casting stones into that pond (albeit local to you) might create a big enough ripple to create some traction for you.