Actually I don't know if it's easy to find or expensive; I don't have any good furniture (half true).
I do have shitty furniture in various grades (but also a few good pieces gifted to me by a now dead aquaintance). Here's a handy list to help you pick out your shitty furniture. They're listed roughly in order of how quickly they will fail and fall apart.
If you have furniture already and you don't know if it sucks or not, pick a joint and just wail on it with a hammer. Go to town. If it's a table you might try WWE-style falls from a ladder onto the table top. What you're looking for is smashing it really good so you can get a look at what's inside.
This is a two part entry, because there's this one thing that's particle board and another thing that isn't, but people kind of use them interchangeably.
The bottom image is Oriented Strand Board - OSB. People call it particle board because it's made out of particles of wood. But it's not particle board. It's larger pieces of wood that are layered with the grain running all whichaways (which resists warping) and it's all laminated and shit. OSB is handy for nailing over broken windows and possibly other things. The major drawback to OSB is that you will never get a very smooth surface without $50 of wood filler and hours and hours of sanding. It's strong, but very utilitarian (read: ugly). Good for building shit in your garage.
I don't know what the deal is with this. It's "Medium Density Fiberboard". I'm guessing it's Particle Board but it uses even finer particles, which is probably desirable in some way. Here it is:
It's heavier than an equivalent thickness of not-MDF. It's also got a ton of binder (glue?) in it which makes it harder on saw blades and makes the sawdust that much worse to suck down into your lungs. What's nice is that it's engineered to within an inch of its life and is basically ready for painting (or not) right out of the box. On account of there's no grain or any distinguishing features.
I think MDF sucks because it's all they ever build anything out of on Trading Spaces. Or that other show... with the surprise room makeover while the spouse is out of town? TLC? Nothing?
Anyway you were pretty much guaranteed a "new couch" made out of an MDF box with a piece of foam on top. That sucks. MDF sucks. I'm going to let my blind ignorance get in the way of professional carpenters making choices on a tight schedule for a TV show.
MDF also really drinks up paint if you do decide to finish it. It's available in pretty much any length and most thickness because, again, it's a completely engineered product. It's also quite strong. And heavy. I already said heavy but it bears mentioning again.
Apparently plywood is kind of a big deal. Here's a picture of some now:
Plywood is actually not cheap. Good plywood - depending on thickness and quality - can cost close to $100 or more for a 4'x8' sheet. But here's the thing. Wood apparently has a bunch of annoying quirks that people have spent like thousands of years trying to combat. It has a grain, and it will split along that grain. It will expand and contract with temperature and humidity and that can do all kinds of awful shit to things that you built out of wood, like break them.
Also while you could conceivably get a piece of wood that ran the length of an entire tree, you're kind of limited on how wide it is by the width of the tree. And then of course the thickness and width of the board you've cut is going to have all kinds of other implications like more splitting and warping and all of the things you don't want it to do.
Enter plywood. Each layer alternates the grain of the wood. It comes in great big sheets and can be manufactured to a variety of consistent thicknesses. Thin sheets can be bent and thick sheets can be made into industrial strength countertops for movie theaters. Plywood can be used for big expanses of wood with little fear of warping or buckling. And it's usually got a finished side that's ready for paint or stain. That's handy too, since it would take a while to sand 32 square feet of wood.
If your furniture is made out of or contains plywood, don't hate it. It's okay. Leave it alone.
If you don't like engineered lumber then here's your alternative, you fucking vultures:
You get a loaf of wood that's roughly log shaped. Wonderful for you to have the time to get out your planes and jointers and build a solid oak conference table for $7,000. Also: Balls to IKEA for making me think at one point that it was well designed, well-made furniture sold cheaply. It's just well-designed furniture made and sold cheaply.
Remember - that crazy looking wood-sheet stuff you see at construction sites is OSB. The guts of your IKEA bookshelf when the screw tears out and you get little bits of wooden confetti everywhere; that's particle board and it sucks.
Plywood is good.