You can buy it from any exchange with high trading volume. It may be better to buy them in portions and withdraw every part after you buy so you do not risk losing the whole amount once
Yes, buying in bulk is always having huge risk. If I am going to buy a very big amount of bitcoin, I will go with localbitcoins but with many sellers at different days.
Hundred bitcoins must be very big amount, so paying for them electronically or through bank is always difficult for me with to respect to my country regulations. So, dealing person to person must be the right solution for my case.