Look at industrial / warehouse supply houses. They'll have rubber and other materials for exactly this purpose. Cut to size if you can't get exact, though 48"is pretty standard. Only issue is how to attach it, but they typically have adhesives for masonry available.
A quick online search is...confusing. The problem is that big/bulky/heavy stuff doesn't ship cheaply, so the online choices tend to be the cheap stuff (that may not hold up so well) or the big box stores (which have limited selection). Just overall guessing, I'd look probably towards either rubber (flexible so it molds to fit well, but may have temperature issues outdoors) or aluminum or steel (very durable, no temperature problem, but may not fit as well with an uneven existing brick floor). Some examples:
Lowes - aluminum
Home Depot - rubber The catch is this a type where it has a part that goes underneath the higher floor. That works great for new installations of tile/floating floor/etc. but not so well for outdoor brick.
In addition to temperature ratings, depending on design there will be a limit (sometimes a very specific min/max, sometimes more vague) of the overall transition height. Some are designed to overlap the two floor surfaces for quite a distance, some only significantly overlap the lower floor.