Mike,
Somebody, somewhere, probably sells those belts readymade. I don't know who, though. If you are not averse to making one yourself, the components are readily available. Belts are probably among the easiest leatherworking projects to get you started, or re-started if you are out of practice.
Dixie Gun Works has a rectangular solid brass buckle to fit a two and one-half inch leather belt. This is item number BG0241, the "Early Brass Buckle" (web link right
HERE). Dixie also has some forged buckles and one enormous brass "Trapper's Belt Buckle" for a 3" belt, if you really want to go all the way. Check out
Dixie's Buckle Page. They also sell a leather strap to fit the monster buckle. It is shown on that page, too.
Wandering Bull and Crazy Crow both sell strap leather in various widths and lengths. I have gotten strap leather from both of them and find it is more supple and considerably cheaper than Tandy Leather. Wandering Bull has it in four colors (white, black, brown and natural) and they have it in the 2-1/2" width you need for the rectangular buckle from Dixie. Here is a link:
Strap Leather from Wandering Bull
However, did people back in the day really wear belts that wide? I think those really wide belts may be a "reenactorism." Rex Norman completed a systematic study of the clothing in Alfred Jacob Miller's mountain man paintings, and summarized the findings in a neat book entitled
The 1837 Sketchbook of the Western Fur Trade:
View attachment 149120
He addresses belts on page 16, stating that "
All belts shown [in the Miller paintings]
are plain leather. No beaded or tacked belts are seen. Belts appear to be no more than two inches wide. Buckles, when seen, are worn to the side instead of in front."
Tom Tobin, who was a sure-enough mountain man, did not read that part about the buckle off to the side, but he did follow Mr. Norman's recommendations for width:
View attachment 149123
Jim Bridger liked a skinny belt:
View attachment 149124
Long Jack, painted by Charles Deas (pronounced "Daze") also got by with a narrow belt:
View attachment 149125
Bigfoot Wallace's belt does not appear to be more than about two inches, if that:
View attachment 149126
Col. Dodge, in this painting by George Catlin, liked his military belt. I think these typically run around an inch and three quarters wide:
View attachment 149127
I know Bill Cody posed for a famous photo wearing a very wide belt, and some of the hired performers in his Wild West Show followed suit. However, they were showmen. I think a lot of today's buckskinners also like to put on sort of a show, which is fine, as this is all for fun. However, if you seriously want to get it right, it looks like a belt of two inches or less may be most authentic.
Best regards,
Notchy Bob