YouTube is probably the place to look if you want to find "Binder Giveaways". In the Canada and USA region, the main company behind it all seems to be GC2B, who sponsor YouTubers like Chase Ross to promote their product. Chase, known on YT as UPPERCASEchase1, is sponsored by GC2B who provides binders, and sex shop Early2Bed, who provides some 'packers' (pretend penises) and offers via Chase's affiliate links, discounts in their online store . Chase typically sends out around 30 breast binders per month and has done Christmas bonus comps sending out 100 binders to entrants around the world.
Last month Chase specified that the giveaway was for youth only. Entrants had to be under 18. The application form has no requirement that applicants have parental/guardian permission, and in fact Chase is well aware that many kids are doing this behind parent's back.
In UK, the company to look for is Spectrum, owned and run by 'trans guys', this company also sends free binders out via many UK 'trans guys' who also hold 'Giveaways' on YouTube.
Comment sections under transactivists' "Binder Giveaway" videos often have questions from kids, under 18, asking how discreet is the packaging, and how to receive without parents knowing, often with other commenters offering 'handy hints' on how to hide both receiving and wearing from parents. I've not seen the YTers discourage this.
In fact, I've never seen a "trans guy" YouTuber tell kids under 18 they must have parental permission, nor ask that they get checked by a doctor to make sure they're medically fit (ie not asthmatic, or with skeletal issues etc) to wear a binder.
Chest binders , or medical compression garments, were originally for post-operative surgical patients, to aid healing. Maybe it is just me, but I see them as medical devices, and I'd think the person using them needs to be medically fit, and capable of being responsible to use the binder according to instructions. I've seen many a teenager comment online about knowing she shouldn't wear her binder for more than 8 hrs, or sleep in it, yet doing so anyway, despite awful pain. I seriously think this is a duty of care issue re: YouTubers sending binders to kids age 12-18. I suspect there are already kids who've been put at medical risk, and I think it won't be too long before we see parents in the media voicing anger that their asthmatic kid had a medical crisis due to one of these people sending her a binder without their knowledge.