I had someone give me 11 Traffic barrels. I had another idea for their use but am now using them with coaching. I just looked online and saw them for sale for as little as 22$ a piece. The site I found them on says the tire ring is sold separately and you don't really need them.
I took about 10 3 ft high Brooklyn Union gas cones while they were being put down on the belt about 45 years ago. I still have them they cost me nothing got the five finger discount on them. My friends were in my car when I told them to take them. Gas company guys put them down and were walking a head of us. They did not see us take them until it was to late. Use your players as obstacles you already have them on the field.
to build mannequins / player sized obstacles, I usually use two (PVC pipe) corner flags and throw a training pinnie over the top to simulate a defender.
This guy is really big on mannequins and provides an alternative- http://www.thetriangletrainingmethod.com/ . I would never buy these but there are some good videos on his site. I'll sometimes use garbage cans or just put 3 cones in a small triangle.
I really don't like today's emphasis on "triangles." It seems like teaching "unchallenged" players to play soccer with crutches. A comparison to "diamonds" is even worse (there is no pivot to steal a Spanish term). The focus is on passing to the nearest player. What happened to combination passing? What about finding the open 3rd man running? What I think people mean by "triangles" is supporting on diagonals rather than perpendicular or horizontal. This is my conclusion from seeing people praise certain static "formations" for having lots of "triangles." Why not just teach players how to be in good supporting positions at the right time? Why did US coaches collectively stop using the term "diagonal?" I want players to see movement patterns and anticipate (soccer is just an 11-a-side keep-away game while looking to score goals), and looking for "triangles" is at best a poor metaphor for that. None of my concerns are for the experienced coach. They are not going to be mislead by how people talk about soccer or coaching. My concern is for the inexperienced coach who may mistakenly focus on teaching players to stay in static shapes on the field.
I have used PVC pipe from the local hardware store. I make them about 3 feet tall so the players can see over while still needing to avoid them.