You're talking about the two that crush between the banjo bolt, banjo fitting and caliper? Yes you can get them elsewhere. I hear you can re-use them but since I could get them easily I wouldn't personally do that.
The stock ones appear to be aluminum but every reman kit I get seems to come with copper. Last time I took the calipers off on my car, I used coppers. Never leaked, never had a problem.
The most important part would be getting the measurement right. If you don't already have the calipers off to measure the washers, here are the measurements I took last time. My Honda Civic uses the same washer part number as yours so these measurements should work with your car provided you have stock calipers.
* 10mm ID
* 15.25mm OD
* 1.25mm thickness
* 2.58mm diameter thickness
Note that while shopping for the washer you don't have to meet exactly these measurements, there's some margin as long as the sealing surface matches. The caliper also gives you significant room around the washer to accommodate different washer sizes.
For example having an extra .25mm thickness or .75mm increase in OD isn't going to matter. If we're talking millimeters it's so small it's virtually unnoticeable.
Here's what I bought last time I did the calipers. I went with these guys since they seemed like they focused only on brakes. The washers were excellent and have not given me any problems. They fit on the caliper exactly without hanging off or interfering with the prongs that stick out that guide the hose. I would buy from them again.https://www.ebay.com/itm/COPPER-CRUSH-WASHERS-10mm-ID/330625585664