If you're in the states look for a local business called a grain elevator or feed supplier. White and red clover is a very standard pasture addition. If you call and ask if they stock bulk quantities of what you want.

Depending on the location they will either stock it or they'll be the exact kind of people who will know who to contact about it.

Or you can contact your local Ag extension officer. Or your Soil and Water Conservation District office. Both of those sources will be able to plug you in on sources of bulk seeds in your area. These folks are also supremely knowledgeable about all things growing and they'll be able to recommend some other techniques to make your green space a benefit.