Gan,
FM is very very subjective. My elder girl gets constipated on Nan and Enfa. So far only Mamil makes her not so constipated.
I used to keep my BM in the fridge for 24hrs. If not used within 24hrs, I will transfer to milkbag and store in the freezer, so i feed my girl fresh EBM at all times. Dun think it's advisable to store in the freezer using the medela bottles, otherwise u will need a lot of bottle! Not sure if they're freezer safe too.
I defrost frozen BM in the fridge (right at the back) and dispense and warm up watever is needed when time for feed. The defrosted BM should be used within 24hrs.
There are many economical brands of milkbags out there. Boots, Lansinoh, Blue Egg, Nanny Milkbags etc etc. Can get online. Check out the bulk purchase threads on this forum.
If u need them urgently, First Years milkbags can be bought from Kiddy Palace, Robinsons etc. But are very ex!!