Where can I buy timing cover & water pump gaskets???

I'd expect any REAL parts store could get them, if they aren't in stock.

or:

http://store.summitracing.com/partdetail.asp?part=SUM%2DG2200&autoview=sku

take $10 to your local parts store of choice and get the parts..
just make sure you tell them they are for something oddball like a 75 Chevy Monza with a 350 in it..
i just like to screw with the kids behind the counter..

Ask for a front cover gasket set. It will have the cover gasket, dampener seal and water pump gaskets. Depending upon the supplier, it might even have the rubber lip seal for the front of the oil pan.