Insteon devices only work with their own hub, they are not compatible with smartThings.
The official “work with SmartThings” lists the Aeotec models. Enerwave and Fibaro are also popular in the community. If you don’t need a dimmer but I just looking for an inwall relay, smarten IT also has options on the official list.
https://www.smartthings.com/compatible-products
As far as where to buy, there are multiple factors to consider. First, is the product description correct and is the device not counterfeit? If you buy from second source places like auctions, both of these issues can arise. Some sellers even intentionally put a less expensive device into the box for a more expensive model, for example.
Then there’s the question of return policy. Somewhere between five and 15% of even big-name devices are defective. So a good return policy can be very important.
Finally, there is the question of technical support. Retailers vary a great deal in the amount of tech-support they can provide for home automation.
We have a deals section in this forum where community members post individual sales they come across. There’s also a thread which lists reputable home automation dealers for both the US and the UK. That’s a good place to start. 