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.