We are all human, so it was probably an honest mistake. It was a learning curve. You and your husband benefited and it worked. All around that is great.
As far as meeting people, dating sites are one method. There are poly dating sites. I do not know them off hand. I know people have had success with OKCupid. Face to face meetings and friends that you may have feelings for. It depends on your comfort level. Are you comfortable with meeting people at a bar or at a lounge? Honesty is important. You may face rejection because some people are just not comfortable with being involved with a married person. It does not mean there is anything wrong with you or how you live, but it does happen. Try not to let that deter you. On the flip side, you never know what a person might be alright with. Everyone is different.
You and your husband can figure out what it is that you both want. Do you want to maintain separate relationships? Do you want to be in a triad where you will have a shared love interest? Would you prefer more of a quad type dynamic where you are involved with another couple? It depends on what you both want. Create a hybrid of any of the above. If everyone's happy and all needs are being met, great!
You guys handled it well the last time. There were problems, but for the most part, I think you are on the right path. Establish limits and sort out boundaries. I know with some things you kind of have to wait until there is a new person is present to say how you may feel about certain things. Be mindful of red flags. Take what you learned from Lia and apply it to now.
Have fun with dating when you reach that point. Make sure you keep your marriage strong by talking and remembering why you love each other. Compromise is inevitable. Patience is so important.
You will find a lot of helpful info on here. We have all been where you are, so rest assured that they are here to help in any way possible.