I had a hard time looking for a sofa due to space constraints - it could only be a maximum of 140 cm wide. I looked at quite a few stores and searched online, but it was either too big or too expensive. In the end, we settled on IKEA:
Solsta two-seat sofa-bed - SGD 179
Stockholm Cirkel throw pillow - SGD 29.90
Assembly and delivery fee - SGD 105 (includes the delivery of the bed)
I must say though that it was not a bad purchase since everyone who has sat on it has remarked how comfortable it is. I don't want to use it as a bed though as the sofa cushions would need to be on the floor. So for our extra guests (those in excess of two), we bought an airbed instead.