With only small minimarkets more or less empty and expensive a trip to Phan Thiet now with 2 big supermarket it's always a good idea.
To get to the new one, LotteMart you need to take the RED bus, the white/light blue bus will go to the other one, Coop Mart, same cost, around 13.000d. The RED bus can be flagged down everywhere, the white usually stop at the designated signs.
LotteMarte is big, one floor is for small shops, one for food and one for clothes and everything else. Price are very cheap (mineral water is half the cost of Ham Tien) but remember that at the cashiers 10% VAT will be added to the total.
Food on the ground floor is not too good, Lotteria is still the worst fast food chain on the planet and the other place has a very limited menu.
The week before and after TET there is a huge local market close to Coop Mart, fixed prices, very cheap, no food stuff.