They're not exactly cheap... set of 4 around $16 from local Honda dealer for me. I'm sure eBay sellers have them as well. But definitely needed in your scenario. Sometimes you can get by not replacing them if you're taking existing headers off and on... some do it for oil changes for example. But if you're installing different headers, then definitely get new copper gaskets. Whenever I pull my header (not that often) I get fresh ones. Not worth fighting air leaks.
If you've never removed old copper gaskets... use a little force with a small flat blade screwdriver or strong pick. They can be hard to see all smashed up in the header port. You may have to use some force. Once you can get one spot separating, they usually pop off in one piece. The fresh ones should just sit in the port with no assistance while you re-fit your header. I've never had to use anything to hold the new gaskets in place while installing headers.
