You'll probably come across sites selling pewter buckles, in your historical costume travels around the internet.
These buckles often look pretty, are quite "period" in style, shape and size, and are frequently cheaper than other offerings online. If you like silver colour, rather than the yellow of brass or bronze, you may decide that pewter is the way to go.
Unfortunately, pewter is a very soft metal. It's not quite as soft as lead, but it is softer than silver (also a pretty soft metal), and it's waaaaay softer than bronze, which was used (sometimes in combination with a copper backing plate) in period for making most forms of buckle.
I know from years and years of experience, both buying (items from other vendors who make things that I don't have the skills to do), and from people walking into my shop at fairs, sometimes in tears, that pewter <em>is not suitable for use as a buckle because it is always under tension and strain</em>. At best, you might get a few fairs or events out of a pewter buckle. At worst, it might not last to the end of the event, or it might last until the second event and bend or snap beyond repair just as the vendor who sold it to you isn't available to take it back and complain to.
That's quite correct! But unfortunately, they are less proof that they were in common use by the public, than they are that pewter was used for the making of counterfeit buckles.
There are records from the Girdler's Guild of London, dating back to the 14th and 15th Centuries, which talk about the guild prosecuting the makers of fake pewter buckles because they were causing trouble for their trade. People were taking pewter copies of the Girdler's more solid work, and selling them as the real deal.
Of course, pewter was cheap, and it broke soon after the unsuspecting customer took it away, and were complaining to the Girdlers Guild about it! The complaints, too, are recorded.
While not as strong as bronze, pewter was used in period for casting buttons, generally for the lower to middle classes. If you plan on wearing your historical costume only occasionally, and you don't do any heavy work in it, then it's okay to go for pewter buttons. If you're likely to catch your buttons on anything (doors, armour boxes, trees…) then the simple answer is don't. Shanks will bend and break very easily.
If you like the look of silver, consider asking whether you can have your bronze or brass buckle plated in silver, so you get the best of the metal's strength, with the looks of silver.
Cote & Cutler Outfitters can do this for you, for a small extra fee.
If you have a question, or you're stuck, we welcome questions, so feel free to ask!
You can even add a photo to illustrate your problem if you like.