Late 13th century knights often wore two belts outside their surcoat, one to cinch the surcoat and and another to hang a sword. The former could be fairly simple, and in at least some effigies of 1250-1300 it's just a braided cord. The latter was usually more decorated, but still austere by 14th century standards, with tooling as much as brass mounts. Pitty there aren't more etant German effigies. At this point in history, not much differed between England, France and Germany armor-fashion-wise though, except that England had some cooler looking surcoats around 1250. It's not until the age of plate that you start to see a gap in technology, where the Germans notoriously hold on to the older styles longer than anyone else. Anyway here's a good illustration of a period English setup:
The most common belts in this period (meaning all belts, not necesarilly sword belts) were made of leather and average about 1" wide. There were wider and skinnier ones, but very wide belts (more than 1.5" wide) were for generally for women, with the exception of sword belts, which were mostly 1.5" wide or so.
Mid to late 13th century buckles were of brass. Around 1250 the common style was a D shape, either with or without a tongue, sometimes lobed with little "blobs" along the curve of the D. The buckle could be attached to the belt either by means of a brass plate that was riveted to the leather, or by running the leather through the buckle to hold the buckle on. This style of buckle lasted until the 15th century. The other end of the belt strap had a brass weight attached to it, commonly referred to today as a strap end. Throughout the medieval period strap ends changed a lot from place to place and by the wearer's personal tastes, but a common example would be about 3-5" long and as wide as the belt, tapering to some simple geometric shape or animal shape or something at the end. Even this early, belts commonly had some brass mounts riveted to them as well. Mostly they were simple geometric shapes or animals, they became more fancy over the centuries. These tiny brass fittings had one or more posts on the back for attaching them to the belt. A hole was punched in the leather, the post pushed through, a square washer of brass sheet was set down over the post, and finally the end of the post was peened to mushroom it, fixing the mount in place. In the late 13th century a knightly belt might have a mount every few inches, or more if the knight was wealthy. Start by choosing a buckle and strap end that are about 1" wide or slightly wider, then choose one or two styles of simple mounts and buy a total of 10 - 12 of them. The leather strap will need to be long enough to wrap all the way around you with all your gear on at least 1.5 times. Keep this length in mind if you decide to just order a belt from somewhere.
Here's a drawing of several 13th century buckles:
http://www.ukdfd.co.uk/ceejays_site/pages/bucklepage10.htmAs with most things about armor, the really good stuff isn't cheap. But a really nice belt hand made by yourself is worth more than a mediocre one bought cheap.
At the top end of sword belt/scabbard quality lies DBK Custom. You will not find a finer sword scabbard produced anywhere in the world.
http://www.dbkcustomswords.com/index.htmlAt the other end you make your own, with parts from a place like Armor and Castings, Gaukler, Quiet Press, or Lorifactor. The leather can be vegetable tanned from Tandy, along with a bottle of dye and a couple simple tools. Veg tanned is not exactly period, but we have to draw the line somewhere.
There are lots of places you can get the parts to make a really nice period belt, let me know if you want to go this route and I can help out more.
Scott