Did R' Hutner zt'l write any seforim that are available to buy? I'm not really seeing anything when searching
He personally wrote and authored two Seforim. Toras Hanozir which you can buy here
He also wrote a commentary on Rabbeinu Hillel's commentary on Toras Kohnim that was anonymous at the time of printing but well known as his sefer today. Most printed editions of Rabbeinu Hillel's commentary on Toras Kohnim include his commentary.
There is also a many volume Pachad Yitzcok on Yom Tovim which he wrote the outline for but was edited by his son in law Rabbi Yonasan David ( allegedly with together his wife, Bruriah, Rav Hutner's daughter) . There is one volume of it that is called Igores U'Ksavim (letters and writings) that were written by him personally, thought he never published them in sefer form.
You can get the Pachad Yitzcok in many seforim stores or by contacting Chaim Berlin directly