I'll second what Corey said about the flat beaters not being interchangeable (probably due to the dimensions of the bowl) from all that I have been able to find. If your mixer is a model K5A (look on the bottom of the base for the model and serial numbers) then the following are the accessories your K5A uses:
K5AB - coated flat beater
K5ADH - coated dough hook
K5AWW - stainless steel wire whip
I wouldn't limit myself to ebay sellers - just go to Google and search on any of the KA part numbers and look for your best deal.
You could also go to the KA website
and click on the "Contact Us" button and ask them what parts you can use on your mixer - but be sure to include the model number from the base of the mixer. Their might be a burnished flat beater that would work - if that is what you want.