Mkkasan has a subway station in Sukongvit (this is not spelled correctly). The subway in BKK is connected with Sky train BTS. I suggest you take the subway to Siam paragon Station and walk to Big C opposite central world for shopping. Siam and central world are downtown, so you can still go there.

Lotus is also found along BTS, which is generally a little far from downtown Bangkok. My final suggestion is that you go to BTS Sky Station and get a map of Bangkok, which shows some big supermarkets, usually at the last stop of Sky Station.

Carrefour has one in sukongvit, which is the subway station between suttizan or Thai Cultural Center. You can ask the hotel waiter.

it's probably not feasible to walk to a big supermarket. Because there are many paths in Bangkok, and it is crowded, if you are not familiar with the road conditions, I suggest you take a taxi, keep the Thai address of the hotel (you can ask the hotel front desk to give you your business card), and tell the driver that you are going to Big C or Lotus. They all understand. You can show your Thai address when you come back, and it's OK. Taking a taxi for 35Bt started, but it was a bit blocked. Therefore, it is a better choice to consider the crown block or subway.