IXOS 214AV 21 Pin Scart plug Gold-plated, Metal body - £ 9.95
Supra Scart plug Gold-plated, Metallised body - £ 8.99
Insert Audio T3 Scart plug Gold-plated, Plastic body - £ 4.95
Van Den Hul also make a very good one, but you may have problems finding it.
If your building from scratch why use SCART - could you not use BNC instead; if you have to stick with SCART I'd talk to John Simm at JS Technology as I'm sure he's tried a fair few chassis mount SCART SOCKETS.