Hi lupo,
In most cases … the foundation that comes with the Beautiful would be suitable and meet the warranty criteria but I have also heard several retailers mention that they were not so impressed with the quality of the Bliss foundations.
If you wish to use a different foundation … then the foundation thread here would be helpful and I would suggest a firm non flexing foundation that had slats that were 3" or less apart (which is better for longer term durability for a latex mattress).
Phoenix