Wedding caterers flexible about specialty goods?

I'm planning a wedding and I picked out a few caterers...I'm trying to not be rude, and I know I'll have to ask them specifically, but there are some items I want to be served that they can't offer (i.e. a special type of oyster) I'm wondering if it is normal for this to happen and if it is generally something they are flexible with or if they strictly serve their foods only for safety issues or profit?