Your choice depends on the camera you're using. The EF lens fits all EOS SLRs and DSLRs. The 18-135 is an EF-S, only for crop-sensor DSLRs (not the older ones before the 20D though).
28-135 on a crop-sensor is a bit of an odd range though - doesn't really go wide. The crop-sensor equivalent would probably be the 17-85 or its successor, the 15-85. The latter is widely praised, the former is widely criticised, though I loved using it. Search the forum, check out reviews (e.g. www.photozone.de)...
Check the Canon Price Guide, you will see the short names for shops carrying Canon lenses. The first posting gives the long name and the contact details. Since you are looking for a Canon lens it helps to focus on those shops where a Canon lens price info is given, right?
Beside this: if you feel brave enough you might also want to look at the less recommended shops along Orchard / Lucky Plaza. They sometimes have older lenses as well. But please do your homework with prices, warranty and the different tactics of 'upselling' with useless rubbish accessories like filters, converters and whatnot. These shops are known con artists, specialized in overcharging tourists.