Your country isn't listed, but I've found (here in Australia at least) that local pawn shops normally have a few XLs and dont mind you checking the firmware yourself. More local online classified ad sites are good also.
I picked up a 4.1 grey XL yesterday from cash converters for $180, one tiny scratch on outer casing but otherwise perfect condition.
If ebay is easiest then I'd look for any that dont come with any recent games and just message all the sellers asking. I came to curse pokemon because so many came with it and you knew the FW would be too high.
If the listing is 'buy it now' and the seller updates the details with it being FW 4.5 or below after you ask, it's likely to be gone really quick.
Search terms that will find a few: '3ds xl firmware', '3ds xl gateway', '3ds xl 4.5', but those will mostly turn up listings from sellers who know why sub 4.5 is valuable and are asking for more money because of it.