I found some cheap terrain on Amazon that happens to be well sized for Guild Ball and was pretty easy to paint. Take a look:
Walls (2 low, 2 high)
Barrels and crates (5 of each)
Chests (4 I think - I didn't get these)
Broken columns (4 I think - I didn't get these, either)
Tree stumps (3)
Of the ones I got, obstacles can be made with groups of the barrels and crates (a few crates, a few barrels, or a combination of both), the low walls, and the tree stumps. The high walls make nice barriers. While I didn't get them, I imagine the chests and columns could be added to either obstacles or barriers, as well.
Here's a picture of all the terrain I made after assembly and priming. Here's a picture of one of them after being painted fully.. Unfortunately, I don't have a group shot after painting.
You can use anything as terrain as long as you define it. You can use a Pringles can and say it's LOS blocking but you can't go on top of it because <insert reason> as long as your opponent agrees.
If you're looking for low cost terrain I have used stuff from this site such as the abandoned buildings. You print them on plasticard and you can build various heights with these. The abandoned building prints out with a choice of 1, 2 or 3 stories and you can print as many or as little as you like. They have access points (Doors) and balconies that you can perch a model or two on to shoot off of and give some cover.
You don't have to break the bank for terrain. What you're looking for is:
If you have those three things, in any number on the play field, it'll be a much more fun game that gives you the ability to maneuver and get into a good firing position.
I also purchased these which were very cheap and work very well for something to hide behind. Because they can be made into a "Box" you can turn them into Ruins for the 4+ cover save as long as your opponent agrees.