After spending the last 12 years living in 8 different rental houses that you can't really alter, I've become a dab hand at decorating with cushions and fabrics. You can really brighten up a plain room by using cheery cushions, and one of my favourite places to get gorgeous (and affordable) Thai silk and cotton cushions is Jim Thompson in Bangkok.
You can find Jim Thompson stores in all the upmarket malls and a lot of the hotels, but I think the best place is their outlet store on Soi 93. The outlet runs over 5 floors and has a huge range of interior soft furnishings and 2-3 floors of fabrics that you can buy by the metre. There's even a cafe on the top floor if you need to take a load off your feet and rest your shopping bags down.
I've always taken a taxi along Sukhumvit Road for less than 100bht, but there's also a Skytrain (BTS) stop that's very close, jump off at Bang Chak and walk back a few blocks and then down Soi 93.
153 Sukhumvit Soi 93
Prakanong, Bangkok, 10260