Similar to impedance mismatches in circuits, electromagnetic waves in waveguides have a similar problem. If a waveguide junction is left open, the impedance of the wave does not match the impedance of free space which cases reflection in the system. To avoid this the waveguide must be terminated.
everything RF has listed waveguide terminations from the leading manufacturers in the industry. You can narrow down on the list of terminations by frequency, waveguide size and power. Download the data sheet of each matching product to review their specifications. Once you narrow down on products that meet your need, you can request a quotation via everything RF.