I usually use one or two tight sports bras (which works for me because of how small my chest is). I'm looking into T-Kingdom's binders because I have heard good things from other gender variant folks I've spoken with.

I most often use T-Kingdom's 801 zip-up binder, which works great for me. I've had to sew it tighter recently because it stretched over time, but it's held together great otherwise.

I bought T-Kingdom's 1500 A-shirt pullover binder at the same time, which I don't recommend. The stitching started undoing itself almost immediately and I've had to repair it several times, and it's extremely difficult to get off if you're even slightly sweaty.

I most recently bought Sohoeva's C4 gray vest binder, which uses the little clasps you see in some bras. It's of somewhat cheap material and you can see the clasps a little through the fabric so I'm hesistant to wear it alone as I was planning, but it works well and is great for the price ($30 and free shipping). The website looks sketchy, but it's linked to a reputable eBay account and I received my binder without any problem, so it's a great place to check out if you're on a budget.