for me the best brooder was cardboard boxes
if you can't get ONE that is big enough, wield the carton-cutter and the packing tape, and use several, put them together to whatever size looks right for you --- you can customize the size to fit wherever you can bear to have the chicks stay for a bit
my seven started out in a small Rubbermaid, graduated to a big Rubbermaid (3 feet x 18 inches) on sale at
Walmart, then to give them more room, we put a couple of roosts in that and added an "attic" of cardboard for them to fly up to when they wanted to exercise
nice thing about cardboard boxes, you can recycle the parts that didn't get poo on them, and compost the parts that did -- when the chicks are done with it all
if you can get appliance boxes, those are great ... mine were mostly boxes that had had big packs of paper towels in them
edited to add --- you can also cut holes in the sides or top, use duct tape to attach "windows" (I got a couple of glassed picture frames from the thrift store)
or -- if you have pets that could possibly get at the chicks, hardware cloth makes a good "roof", weight it down ...