if you want real pearls (deep sea collected pearls, GIA called them saltwater natural pearls), you must need to buy vintage with GIA. if all 5mm I guess over 0.1x-0.2m.
In these few decades no one would dive deep deep under the sea for 1 pearl.
for new collectors, can start from 2mm-2.5mm (very tiny), costs around $30000-40000 with GIA.
You can get these from online USA or Australian dealers, or try jewerly fair in HK.
for bigger ones over 5mm (e.g. 7mm-9mm) may cost up to 1m.