Having just done this for our 6 year old, there are many places to get them from, costing anywhere from £ to £££££££££££££££££££££££
We got ours from Atlantic Trampolines. I settled on this as quality of the kit is very good whilst still being £200.
Ours is a 14ft “Sunken” trampoline, though we opted to not go the full sunken route and just have sitting approx 6″ above the ground. This does mean digging a hole, which is no easy feat when it needs to be 12-13 feet wide by a meter deep.. Anyway, most websites have blogs of how to set them up
Look at AtlanticTrampolines, or CapitalPlay they both have blogs on there and videos.