Crumpler~Raffles City.

One place worth to check out is mustafa center. This place has huge range of variety on bags. The prices in mustafa center are reasonable.
Avoid going there on weekends, as it gets too crowded on weekends.

You can get them at courts or popular or any shops that sells laptop should carry them.

P.S= Please put as best answer if you think that my answer is useful thank you.

Ok if you are near Orchard Road, go to Ngee Ann City - Takashimaya Shopping Centre. Basement 1. There is a small place near the event hall where they sell good, stylish designs which can be used for both work and play. These bags comes in various sizes to fit your laptops.

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sim lim and funan would have a variety. ishop in cine has a few.. elife @ suntec has some.

or you can search for customisable ones online such as fabrix.com or other overseas sites like etsy.com

you can also try BONKERS shop in queensway shopping ctr. the uncle is very friendly and has quite a number of limited edition designs from good bag brands.