When do you need the kebaya? Hari Raya Puasa is starting end of the month, there would be a geylang bazaar with a wide range of kebaya on sale. You can check there out.
You may want to go to Tanjong Katong Complex and check it out. I'm not so sure about the pricing. However they have a lot of selections there.
East coast has stop houses which sells them. Alternatively try geylang Serai too