Kaffir lime leaf syrup
Kaffir lime syrup is very practical to have on hand to improve cocktails, to top mango ice cream or to season summer drinks.
Unfortunately it is very difficult to find. The best is Royal Kaffir Lime Leaf 500ml. Cane sugar leaf syrup. https://shop.bluekoff.com/ProductList.aspx?cat=113&subcat=113&id=4356
par The Silpin Thailand
This excellent producer also has a line of liquid aromatic toppings for cooking which they call edible flavors. Unfortunately these products can't be found outside of Thailand.
So you'll just have to make your own kaffir lime leaf syrup.
A lot of recipes recommend cooking the leaves in a syrup (50% sugar 50% water) for between 5-12 minutes then letting them rest for up to 4 hours. 3 leaves bound together for 1 cup of syrup. Thai sites say to infuse at a low temperature.
It's a good idea, around 60° this way you keep all the essential oils.