Are you looking for increased performance or just a stainless steel replacement exhaust?
If you are just looking for a stainless exhaust system then you should be able to have one made at one of the generic stainless steel exhaust companies such as Powerflow etc. Most of these types of companies will merely just replicate your existing system in stainless steel and are unlikely to give much in the way of power increases as their products are not based on any form of testing or research.
If you are looking for increased performance then CSC are your only option, Supersprint/Sebring etc have long since stopped producing parts for the 33. A system from CSC shouldn't be as expensive as £1000, it would cost around £600 depending on where you buy it from, I would recommend you try here:
Alfa Romeo 33 Tuning - Sportauspuff - Alfa Romeo Shop - Tuning, Styling, Fanartikel und Modellautos
or you could contact EB Spares http://www.ebspares.co.uk
. They don't supply 33 parts but they are CSC distributors and if you give them the CSC part number they will be able to order anything you are looking for and their prices are not too bad.
Part numbers can be found on the CSC website: CSC Marmitte - Terminali
and you can order directly from the factory if you contact them.
I presume your car has a catalytic converter? If you fit the CSC cat manifold then you will need to find a replacement catalytic converter because the manifold is designed to fit with the early European 16v cat which we did not get in the UK. It differs from the UK cat in that it is much shorter and has a 3 bolt triangular flange rather than the later UK spec cat which has a longer pipe and a 2 bolt oval flange. I have the CSC cat manifold and assumed that there was an error with the design and it was only after a lot of searching on Dutch and German forums that I found out about the earlier type cat. I then had to try to find one and eventually imported one from Denmark but it was only around £150. If you want to de-cat your car then you could just get the CSC non-cat manifold and CSC centre section but you'd need to re-fit your original manifold and cat for the MOT, something I couldn't be bothered to do.
For your interest here is the CSC manifold, I bought mine before they switched production to stainless steel so mine is mild steel. Also a picture of the shorter European early cat you would need.