I've bought a couple of unlocked phones recently, and my experience has been mostly positive. One great place to start is directly from the manufacturer's website, like Apple, Samsung, or Google, because you know you're getting a genuine product with a solid warranty. Amazon and Best Buy are also reliable options, as long as you purchase directly from them or verified sellers with good reviews.
When buying, ensure the phone is compatible with your carrier's network frequencies. Checking for reviews on durability, software updates, and battery life can also help in making a good choice. In terms of brands, it often comes down to personal preference and ecosystem compatibility. If you prefer Android, Samsung Galaxy and Google Pixel phones are consistently well-rated. If you're in the Apple camp, an unlocked iPhone is a solid choice.
The main downside can be the up-front cost compared to contract plans, but the flexibility and freedom from contracts often outweigh that. Make sure to double-check return policies before committing, just in case it doesn't meet expectations.