Brass wont rust or gull, thats why they use brass; There's lots of Industrial Fastener places around (up here we have Fastenal) and they'll be able to match up just about any bolt for you (if not they can get it) and usualy at a fraction of the cost! But if you question the quality of aftermarket brass bolts, stick with the stock ones... if you use antiseize paste and keep the threads clean, they should last a long time. Problem is that a lot of places that do service wont take the time to clean threads and use antiseize as to them time is money
For gaskets, sensors and stuff, its also worth checking out Autopartsway.com , their prices are good (for instance 8 something for the header gasket and 2 something for the exhaust gasket) Most of the stuff is manufactered by the same company as that sold by Porsche so your getting the same part - minus the Porsche sticker markup.
For Canadians they have a canadian site autopartsway.ca and shipping is always domestic so no duty etc to deal with (so far never had any issues) actually the last few ordered parts like sensors came in the genuine Porsche bags/stickers.
Anyhow just a suggestion...