You can make an "Idea Buyer" by doing your branding, online marketing and advertising about your services.
Firstly I suggested , if you can one product selling in market so that firstly you can think what about need for client in market?. You can big search in market and take the information after you can made for your product and you can advertising online.
Buying ideas is not easy. There's alot involved and often results in having to have proper lawyers/attorneys based in the specific field. Nothing can protect an idea you buy from being stolen , even lawyers. Also, an idea is useless unless you can show execution. Most people who are interested in buying and selling ideas need proper platforms or people to help show the practical side of the idea and the use of application.
Post an ad for ideas wanted, I have seen this done in the past, even before the internet became big. Just the other day I found an ad online by someone that wanted to sell his idea. The point is people are out there buying and selling ideas, you just need to search for sellers. IMO
The biggest trouble you might face is pricing the idea for buying them. But there are ads out there I have noticed to look for ideas, so start from there.