Hi,
you have now found the downside to owning a Citroen...not the only high price dealership, but there you go.
It is certainly possible to get one from a breaker's yard and fit it yourself. Best bet is to buy a workshop manual first (mine came from Haynes) which are widely available. A slight word of warning...on Citroens, removing the dash is not a five minute job, but with the manual you will get step by step instructions and, apart from the mileage ending up different (try to get one that doesn't have a massive mileage recorded) which may affect the resale value when you do come to sell it (though keeping the old one to show your prospective buyer will help overcome 'high mileage' knockdowns)
I own a Citroen Xantia, and have found the dealer's workshop prices very high indeed, so I know how you feel.
Good luck,
BobSpain