“You require a unit that outputs DC 5V with at least 1 Amp. The tip must be 2.1 x 5.5mm and centre positive.”
Here’s one that’s UL and CE listed. I’m guessing you’re probably concerned about a cheap chinese power supply nuking the OSSC. The UL and CE listing provides some level of confidence. Others might have better “known good” suggestions, I just found this with a quick search.