Old Weller soldering station tips
Your tip is the Weller PTK7. The 7 denotes the max temperature which is 700F. A higher number, such as 8, denotes 800F. 6 denotes 600F. 

Given you're from France, I'd try ebay.fr for these types of old tips. The original tip can be found here, with origin from France:
Ebay - Weller PTK7

If you want a smaller sized tip, things become more complicated. But my guess is that the Weller PT-O8 should work perfectly in your model: 
Weller PT-O8 - Ebay

It seems to have the same diameter and lip as the PTK7. You'll have to test it to be sure.