Yeah I agree with Erkenstein. 30% till 12 weeks then thin it. Also ask your feed store if they can get you what you need ordered. Most feed stores will just add a couple bags to their next order for you.
I get them onto a 21% feed eventually. I'd prefer higher protein but I'm working with a local feed miller who uses a non-GMO broiler mix and adds limestone for calcium. They've done fine with it.
Some people use higher protein during the birds' entire lives. There may be more risk of kidney problems since the kidneys have to work harder, but the birds have pretty short lifespans so I haven't heard of any definite issues presenting.