The skirmisher boots are a pair of ranging boots from the skirmisher armour. It is a common prize from the Treasure Hunter. Notably, the fifth tier gives better stats than both ranger boots and spined boots. As they have combat stats, Diango charges a fee of 50,000 coins to recover them after being destroyed.
To change the tier of a piece of equipment, it must be in your inventory and not worn; then right-click it and select the Change option.