Yoram, a telltale sign Polydomes are cactus is yellowing of the diaphragm, drying and cracking. I wouldn't recommend replacing the 3" Polydome midrange diaphragms in your Kappa 7's with the 2" Polydomes from the RS-5B as the power handling and dispersion pattern of the 2" Polydome will be different and may upset the balance/coherence of your speaker, and may result in the driver prematurely failing. You might be lucky and find an original Infinity diaphragm or driver on Ebay in good nic, but you don't know what you're getting and unfortunately Infinity's demise has given rise to a number of unscrupulous sellers selling often misrepresented or damaged parts at ridiculous prices.
Unfortunately replacement drivers (part #902-3075) have not been available for some time and Orange County Speaker Repair no longer have stock of replacement diaphragms (part #10-239), however they sell a generic replacement driver which is a drop in replacement (part #02-239) for around $130. My advice would be to have a look at their website. If you drill down into that part there are a number of user reviews which offer good advice about installing the drivers. Good luck and let us know how it goes!
Unfortunately replacement drivers (part #902-3075) have not been available for some time and Orange County Speaker Repair no longer have stock of replacement diaphragms (part #10-239), however they sell a generic replacement driver which is a drop in replacement (part #02-239) for around $130. My advice would be to have a look at their website. If you drill down into that part there are a number of user reviews which offer good advice about installing the drivers. Good luck and let us know how it goes!