I've bought from all three of those sources with no issue. They are all good for different applications.
Spotlight is great if you swap stuff out often as it is front loading and you don't have to take the frame off the wall.
However, for modern 27x40, there is a frame at Michael s that can be had for under $30 when they have their sales. However, they tend to look cheap.
Michael's also has a standard size that fits half sheet posters that looks nice and is cheaper than the dedicated ones.