MOSFET with *actual pin* for body

The parts on that schematic are very low capacitance (and, unfortunately, that generally means relatively high 'on' resistance) compared to the otherwise nice ones that Ignacio mentions in his comment.

The switch is designed to have low charge injection since that directly affects the accuracy of the nulling.

You might consider something like the ADG5236 in a modern design, which has 0.6pC of charge injection, which is only 60nV on a 10uF capacitor.

Or use an amplifier that has the autozero circuitry internally, but I see you're actually working on a sample-and-hold circuit.

The 4 pins shown inside the MOSFET symbol are a representation of the physical structure of the MOSFET. However, for the vast majority of MOSFETs, the substrate is an integral part of the MOSFET and can't be separated from the source.

I have seen low-current MOSFETs that DO have the substrate come out as a separate lead but I haven't seen any of those available for several decades. Doesn't mean that they don't currently exist - I just have never needed to find any.

Most people use J-FETs such as J112 (N-channel) or J172 (P-channel) for doing the kind of switching that you are trying to do.

Take a look at the ALD1115 or ALD1105 from ADVANCED LINEAR DEVICES, INC. It'a a complemantary MOSFET N/P pair. V+ is connected to the substrate, which is the most positive voltage potential. V- is connected to the most negative voltage potential