Grove connectors, cables, and sensors are actually quite easy to get. Usually when you buy a Grove sensor, it comes with a free 20-cm cable. Or you can get 5-packs of the cables for around $3. https://www.robotshop.com/en/grove-4-pin-buckled-20cm-cable.html
You can get 10 Grove connectors for $1: https://www.robotshop.com/en/seeedstudio-grove-4-pin-connector.html
Here’s a Grove version of HC-SR04 ultrasonic sensor: https://solarbotics.com/product/29062/
Also available here: https://www.epictinker.com/Grove-Ultrasonic-Ranger-p/sen10737p.htm
And from Jameco: https://www.jameco.com/z/101020010-Seeed-Studios-Grove-Ultrasonic-Ranger-Distance-Sensor_2237845.html
Solarbotics.com and Epictinker.com both have a bunch of Grove sensors and cables.
RobotShop has over 100 Grove-related products: https://www.robotshop.com/en/catalogsearch/result/?q=grove&order=relevance&dir=desc
Robotmesh has even more: https://www.robotmesh.com/catalogsearch/result/?cat=0&q=grove
Mouser sells a variety of Grove/Seeed products: https://www.mouser.com/new/seeedstudio/seeed-studio-grove-sensor/
So does Digikey.