It's not obvious at all to me that 'a few' nV/sqrt Hz noise swamps your signal as you have said nothing about bandwidth. If your bandwidth is very low then there may not be a problem. Note that it is bandwidth not maximum frequency.

Note that the quoted nV/sqrt Hz noise is above the 1/f corner frequency and if your frequency is low then you may have significant contribution from the 1/f noise as well. Chopper amplifiers have much less 1/f noise but often suffer from relatively high white noise.

A lock-in amplifier, a standard piece of kit in many labs, effectively has a very low bandwidth because of the synchronous demodulation. By modulating and demodulating, in some circumstances, you can operate in the white noise region of your amplifier (constant nV/sqrt Hz) rather than at the lower end.

If the signal is above some tens of Hz, and the source impedance low, you can get a boost by using a simple step-up transformer at the input. There will be a Johnson-Nyquist noise contribution from the winding resistance, of course. The transformer with 1:n turns ratio decreases impedance by 1/sqrt(n) and decreases noise by 1/n, ideally.

It's also possible to build an arbitrarily low noise amplifier simply by paralleling 'n' low noise amplifiers and summing the outputs. The input impedance decreases with 1/n and uncorrelated noise decreases with 1/sqrt(n), so 100 amplifiers in parallel would have 1/100 the input impedance and (ideally) 1/10 the noise.

If you happen to have a liquid helium cryostat and some DC squids available you can get much lower noise levels but your budget won't pay for even a single cable let alone the setup.

It's not obvious at all to me that 'a few' nV/sqrt Hz noise swamps your signal as you have said nothing about bandwidth. If your bandwidth is very low then there may not be a problem. Note that it is bandwidth not maximum frequency.

Note that the quoted nV/sqrt Hz noise is above the 1/f corner frequency and if your frequency is low then you may have significant contribution from the 1/f noise as well. Chopper amplifiers have much less 1/f noise but often suffer from relatively high white noise.

A lock-in amplifier, a standard piece of kit in many labs, effectively has a very low bandwidth because of the synchronous demodulation. By modulating and demodulating, in some circumstances, you can operate in the white noise region of your amplifier (constant nV/sqrt Hz) rather than at the lower end.

If the signal is above some tens of Hz, and the source impedance low, you can get a boost by using a simple step-up transformer at the input. There will be a Johnson-Nyquist noise contribution from the winding resistance, of course. The transformer with 1:n turns ratio decreases impedance by 1/sqrt(n) and decreases noise by 1/n, ideally.

It's also possible to build an arbitrarily low noise amplifier simply by paralleling 'n' low noise amplifiers and summing the outputs. The input impedance decreases with 1/n and uncorrelated noise decreases with 1/sqrt(n), so 100 amplifiers in parallel would have 1/100 the input impedance and (ideally) 1/10 the noise.

If you happen to have a liquid helium cryostat and some DC SQUIDs available you can get much lower noise levels but your budget won't pay for even a single cable let alone the setup.