Tyras Camp is in a secluded part of Isafdar, the elven forest. You gain access to Isafdar and the camp during the Regicide quest. You can buy every type of halberd (with the exception of the white halberd which can only be bought in the White Knights' Castle) in a store there, including the dragon halberd.
The easiest way to reach Tyras Camp after the Regicide Quest is to go to Catherby Port and charter a ship to Port Tyras, which will cost you 3,200gp.
Note that if you have reached Tyras Camp in the Regicide quest, you can still buy all the halberds from the store.
There is one general store located in Tyras Camp, which only carries halberds. Prices are listed below:
The amount in stock may affect these prices.