To buy a truck overload sensor, you can do it in the following ways:

1. **Online shopping platform**: You can search for truck overload sensors on major e-commerce platforms such as Taobao, JD.com, Alibaba, etc. There are many sellers on these platforms that provide various models of weighing sensors and related equipment.

2. **Professional weighing equipment company**: You can directly contact professional weighing equipment companies or manufacturers, who usually provide more professional products and customized services.

3. **Auto parts store**: In some large auto parts markets or professional auto parts stores, such products may also be sold.

4. **Industry exhibition**: Attend relevant automotive industry exhibitions, where there are usually various latest vehicle-mounted weighing technologies and equipment on display, and you can learn about product information and purchase them on site.

5. **Ask familiar logistics companies**: If you know a logistics company, you can ask them about purchase channels, because they may have fixed suppliers.

When choosing a truck overload sensor, you need to consider the following factors:

1. **Accuracy**: Choose a product with high accuracy to ensure the accuracy of the weighing results.

2. **Installation method**: Consider whether the product's installation method is simple and whether it will damage the vehicle body structure.

3. **Durability**: Consider the durability of the sensor, because trucks often travel under different road conditions and the sensor needs to be able to withstand harsh environments.

4. **After-sales service**: Choose a supplier that provides good after-sales service so that problems can be solved in a timely manner when encountered during use.

Please note that before purchasing, be sure to confirm whether the specifications and functions of the product meet your needs, and whether there are corresponding quality certifications and safeguards. At the same time, understand the product's usage and maintenance requirements to ensure the long-term and stable operation of the equipment.