SportsKnowHow.com - Rugby Field Dimensions Diagram
Here, the goal post size is on there. Just make em yourself. It shouldn't cost that much.
Edit: I see you need to know how to make them. If you can find a joint (don't know the actual name) like a cross without the right arm. Buy 2 of those, and 5 pipes, make sure all of this fits the size I gave you. You might need the bottom pipes a little longer than 3 Metres so you can secure this in the ground. Take two of your pipes, put one pipe in the bottom of the joint, and the other on the top. Do the same with the other side, and use the other pipe for a crossbar. Then dig 2 holes a little wider than the pipe, put it in the holes, and secure it with the dirt. If you want a regulation rugby field you'll have to do this two times. One on each end.
It's kinda hard to explain, but I hope you understand.