Hi Ben, in case you don't know chipbreaker is an alternative name for the cap iron. Chipbreaker is more commonly used on the other side of the pond.
AFAIK the Record Marples is a fairly standard Stanley Bailey clone, so any cap iron from a Bailey clone plane with the same width cutter should fit yours. The no. 3 is unfortunately not the most common size, cutter width is 1 3/4", so finding one won't be as easy as finding a replacement cap iron for a no. 4. Normally I'd suggest trying a few car boots to see if you can buy a cheap donor plane but I've never seen a no. 3 in the wild in Ireland so that might ultimately be a fruitless search.
The rusty old ones you saw on ebay, if the price of any of those suits you then don't let the rust bother you too much. It's quite simple to return a rusty tool to serviceable condition and even what looks like very heavy rust can make very little difference at the end of the day.
With a cap iron you would probably want to fettle the front anyway (undercutting the leading edge and making the front angle steeper) and that will naturally deal with any pitting there might be in that area. Cap irons are made from mild steel which is much softer than tool steel so this is very easy work and shouldn't take longer than a couple of minutes, either using a file or whatever sharpening media you use.
