The sun works by fusing hydrogen to helium (at the moment). While hydrogen is a single proton (I'm ignoring electrons here… not sure if they are relevant since it's probably a plasma anyway) helium consists of two protons and two neutrons.
To get fusion to work on earth we therefore use deuterium and tritium (getting us one extra neutron in the process, but it's easier to fuse them).
I don't think that the hydrogen in the core of the sun is mostly deuterium (or tritium for that matter). So where do the neutrons in the sun come from?
Best Answer
At high enough temperatures (or in other words interaction energies) you can produce pretty much any particle. This is also the logic behind particle colliders that allow us to produce various other particles by colliding protons or electron-positron. This can explain how the seemingly non-existent neutrons you are referring to suddenly appear. To be exact the weak force allows neutron formation from proton only initial states.
The sun's surface temperature in natural units is $T_\odot = 0.46 \text{eV} \ll 1\text{MeV} \approx m_n - m_p$ which makes the production of neutrons or more importantly Deuterium pretty rare. However, through proton-proton chain interactions Deuterium can be formed. Once you have Deuterium you can fuse it to produce Helium.
This of course is just the tip of the iceberg, the full dynamics of the sun also include Tritium production amongst other processes.