Any type of resonator that meets the 1.5% tolerance (15,000 parts per million) required by the USB specification can be used for a low-speed device. There are many such resonators available in the market.
As the CY7C63413 datasheet indicates, there is an internal 30pf cap at each xtalin/xtalout pin. In an AC analysis, this will give you an effective 15pf capacitive load. There will typically be 2-3pf of stray cap in the logic. Therefore, the total capacitive load that an oscillating element sees is about 18pf. The 18pf load cap is a common recommended load cap for many crystals in the market. So, you should choose a crystal that requires an external 18pf load, but note that these caps are integrated on chip.
You may want to take a look at the ZTA 6.00MHZ (this is what we use in-house).