The mode knob is part number RN-117B, black knurled knob with chrome skirt (24 x 14mm). It is available from a variety of suppliers (with slight variations). Search for "RN-117B knob".
The four small knobs are 19mm x 14mm knurled knobs with silver center. The knobs are different depending on when the Context was built. Knobs for early Contexts can be purchased from Small Bear Electronics, SKU 0811B-BLK. Later Context V1s used RN-206F knobs, which are available from a variety of suppliers.
When replacing the small knobs, we recommend you replace all four to ensure a perfect match.
All of the knobs are designed for 1/4" round potentiometer shafts and have set screws for fastening.