I think your best bet is to forget the internet.
Pick up the phone and contact your local law association - the professional body for lawyers. If you approach them right, they will probably be happy to help you if they see it may benefit their members.
A few years ago when I was developing a service for the accounting industry, I presented my idea to a group of 30 accountants during their monthly professional development night. They gave me lots of feedback and filled in a survey for me. I also got cards from those who were interested in using my service once it was operational. We got our foot in the door with a friend of a friend who belonged to the group.
Most people are happy to help if they are approached personally.
(I'm also wondering whether lawyers would frequent forums etc. Might be too worried about any legal liability arising from what they write?)
I have had success looking for specific types of user with my LinkedIn account. In your case, you can search for a type of industry (law) using the Advanced Search. You can request they join your network, which is unlikely, or you can upgrade your account to allow you to send InMail which is essentially like an email to the person. You can categorize the cost of upgrading your account as a business cost.
Also, probably the best option, is to search for and join groups on LinkedIn that pertain to law and attorney's. You are able to post comments there and are likely to get feedback from those willing to discuss product.
i just wrote a comment about your startup costs. If you are marketing to lawyers and and law firms then you have an advantage because you can purchase a marketing list.
here is how i would approach it.
1. email them.
2. Follow up phone call.
3. postcard or direct mail
4. follow up phone call
Bottom line TELE marketing works best, but you need an excuse to call. If you give them an email you can simply call them asking for feedback.