Sites are generally managed by local clubs who negotiate access with land owners. Information on their sites can be found on individual club websites. Please check the guides and member or visitor access conditions before visiting sites. Some clubs are compelled by landowners to ensure pilots are members or have temporary membership. Others may allow visiting members to fly their sites or pay a nominal day fee.
For an indication of where sites exist, direction they face and wind conditions Paragliding Map is a useful site
Most clubs also have Facebook pages which are also a good way to make contact before a visit.
But please remember to check membership conditions and seek out a club coach to give a site briefing
I have also found it useful to look at sites on youtube or vimeo to get an idea of takeoff, landing zones and the topography. In some cases you will even find site guides