Spring tips for these are sold at prices around gbp35 for five (PicoTech). They are 0.65mm dia and 16.5mm long
I wonder whether a P50-B1 pogo pin (e.g. Harwin P13-0123) would fit these (nominal 0.68 dia 16mm long). If so, they're available for about gbp7 per hundred, which is a LOT more reasonable!
D.