Hi Buffalo, NY
The bedframe is an important part of the sleeping system and is the part that supports everything else (the foundation, the mattress, and the people on the mattress). In the case of a steel bedframe I would make sure it has at least 1.5" angle iron and good center support to the floor (preferably with at least two legs for king sizes) and legs that don’t bend or collapse when you move the bed. Most of the major bedframe manufacturers make steel bedframes that will be fine but in general I would avoid the cheapest versions which can be weaker and not as solid or well constructed. If you (or the mattress) are in a heavier weight range then I would look at more heavy duty frames.
If you are considering a wooden bedframe then I would make sure that it is strong enough to support the weight of the mattress and foundation and the people sleeping on it (the manufacturer should be able to tell you about any weight limitations) and has at least 5 hardwood cross slats and that it has a center support beam (in queen or king sizes) with good center support to the floor.
If you plan to use a headboard or footboard I would make sure that it can be easily attached to your bedframe. Some headboards are bolt on and some are use a hook type clip on (which may need an adaptor) and some frames don’t allow for a footboard to be attached if you are considering one.
Leggett & Platt doesn’t sell to consumers directly but they are a major supplier to most retailers across the country where you can buy their products (frames, adjustable beds, foundations, boxsprings etc). Their frames are on one of their websites here.
There are also many other good quality and strong frame suppliers however such as Knickerbocker (they make one that is very strong called the monster bedframe) along with Mantua and W. Silver Products and Glideaway and Hollywood, and Malouf Structures to mention just a few so I wouldn’t spend too much time on these when almost any retailer will have a good quality and suitable frame or a heavy duty frame (for heavier mattresses or body weights) and if they don’t they are widely available.
This thread also has more about heavy duty bedframes.
This Beducation video about bedframes is also well worth watching to get a good sense of what to look for.
Phoenix