Most low end wireless routers priced between (2000 and 3000) will now come with non-detachable antennas. Linksys used to have very good routers in the form of WRT54G and WRT54GS but now their new versions use a different chipset which is not compatible with any 3rd party firmware. The older versions will be very difficult to obtain. ASUS has some good wireless routers but you will not get them in Eastern India and I have no details of any vendor having them in the other regions of the country but I have
heard that they are available.
If you are so adamant on using a WiFi link, then you can get any D-Link router and use the antenna suggested by NetFreak and Samurai. Infact you can contact Neetu of Sigma and ask her if she has any antenna which meets your requirement.
I suggest you take a look at
Wireless LAN and decide. Then you can inquire about the availability and price from Sigma. They will be more than willing to courier the products to you. Take my reference and ask for a discount (They know me as Sumit Bajoria from Shillong). Generally she waives off the taxes and courier charges if you pester her.