I just bought a new mattress. I can’t find a bed frame with slats online where the slats are 3’’ apart or less for a queen bed. I’ve been looking all day. I’ve looked at everything from Amazon to expensive furniture stores. My mattress was so expensive, I want to be sure it doesn’t sag due to the slats being too far apart. What are people buying that works well, is affordable and doesn’t sit too low to the ground?
Hi gmari, and welcome to the Mattress Underground 
First, I want to mention that you should always check with the mattress manufacturer to make sure any bed frame or platform you use is approved, so you don’t void any warranties or return policies, as wll as to make sure the platform will support the weight of the mattress.
Surprising that you have had trouble finding slatted frame with the recommended 21/4"-2 3/4" slat spacing…have you looked at the Zinus Alexia on Amazon? A number of the Trusted Members of the site carry slatted wood frames, depending on what you consider ‘too low’: APM has slatted foundations at 8" height, and Luma carries a similar wood foundation at 8", and SleepEZ has their natural wood foundation, and all have adequate slat spacing to maximize the air flow, but also sturdy enough to support the mattress and sleepers. Also make sure there is adequate central support (a center beam or central support legs on a raised frame) to distribute the weight of the mattress more evenly.
~ Basilio