There is no separate development kit for the nRF52811 chip. Projects for nRF52811 can be developed on the nRF52840 DK.
For information regarding the differences between ICs in the nRF52 Series, see the comparison table on the nRF52 Series page.
For more detailed documentation regarding nRF52811 development, please see Developing for nRF52811 in the SDK documentation.