if it's not in stock, it's most likely on order.

"On stock" sounds strange to my BE ear!
I would definitely say "in stock".

Thank you, that's exactly what I thought!

The person may have been thinking of 'on hand.'

However, it could be that 'on stock' is also used in colloquial AE, even though 'in stock' is definitely the standard way to say it.

It's one of those things that once you start to analyze it you can't remember what you've heard and whether it sounds right. (-:

However, the sentence has a couple of other problems:

We often have strollers at reduced prices; simply give us a call and we’ll tell you what we have in stock.