You can buy hardware from many vendors. Maybe Dell or HP(E) websites are an easy start.
You also need a good place for your rack, that will generate a lot of heat and noise. Not every closet is a good datacenter.
You are looking for a competent sysadmin I believe, but good luck to attract one with such a small setup. It’s probably not worth the cost anyway, and perhaps a few existing people in your team should do it themselves.
What you want to do is very common in research labs, where people are competent, motivated, and with more time than AWS budget.