Hi little_ones,
You can get the maternity pants at Kiddy Palace. If you want to get cheaper ones, you can go to Chinatown, those shops above the market. The maternity clothes there are much cheaper and sometimes you are able to get the same designs that Kiddy Palace is selling at a much cheaper price. Another place you can go to is a shop on the 2nd level at Tanjong Pagar Plaza. The clothes there are very nice too. From what I know, some of the pants do come in sizes, eg s, m, l. However, some just have the elasticity and no size.
If you feel that your clothes are getting tight, then change. There is no hard and fast rule as to when you can start wearing maternity clothes. For my previous pregnancy, I only started wearing maternity clothes in my 4th month.