Due to the expensive angular interdental toothbrushes sold in Spain, I've made an adapter for the straight version, which are cheaper. It's made for the Auchan brand, in Alcampo supermarkets. You only need to cut the wire of the brush using a scissors and then insert it in the hole of the adapter, it fits with the sufficient presure.
When it breaks you can extract it and cut another brush, and you can keep using the same handle.
This solution involves an interesting save of 6€ per month.