There's loads of exercises you can do to help strengthen & build your neck, some of these will probably get flagged up by some as unsafe, but these are ones I have personally done for years with no ill effects (yet!) and most of them can be done anywhere with little to no equipment.
1 . Wrestler's bridge - you can do this with both belly facing up and belly facing down, google it or youtube it and you'll get what I mean.
2. Isometric neck exercise - front back and sides, you can use your hands, a towel etc to put resistance on your head and fighting it.
3. Lie on a flat bench facing up with your head and neck hanging off the end and place a weight plate on your forehead, tuck your chin in and hold, or alternatively nod your head back and forth (keep hold of the weight, you don't want it slipping off!)
4. As per # 3 but facing down and with weight on the back of your head.
5. The neck harness - strap it round your head and attach weights to the other end and away you go.
Google most of these for more detail - I'm sure there's more, but these are ones that I do or have done.