I usually buy in Lei Tung and South Horizon's ParkN Shop. Lei Tung's market is big but the floor is wet, I bought once in here only. But you can buy flowers in Lei Tung's market in Chinese New Year. South Horizon's ParkN Shop is very big and plenty of goods and you can use its delivery service.
If you want to goto Jumbo, it has 2 entrances, one in Sham Wan, the other one in Aberdeen (exactly the location said by monica). I tried to walk to Aberdeen entrance, if the weather is not very hot, it's OK.
We normally go to Aberdeen Centre for grocery shopping. You can take the mini bus 39C there (only $2.5). You can take the same bus back just opposite the Big Park n Shop. There are also Watsons, Pricerite, public library, Japan City, (it's just like a small town centre) and lots of eating places as well. To take a cab there, costs only $15. But I will buy some stuffs back from Causeway Bay if not a lot. cheers