The word portmanteau is actually one of those lovely words that means exactly what it says--it's a combination of the French port, "carry," and manteau, "cloak." Which is essentially it's function! In general, the word just refers to a suitcase, but in a historical context it tends to be a little more specific.
In the 18th century (though I've seen examples up to the 1830s), portmanteaux were often cylindrical or sort of curvy-oblong in shape with a flap that buckled over the opening. I'll be the first to admit that I haven't done a ton of research, but the look is very iconic and (to me, anyway) recognizably period. And, I've coveted one for absolutely ages.
Imagine my delight, then, when Samantha and I stumbled across these two beauties at the Kalamazoo Living History Show this year. They were being sold secondhand, so we got amazing prices, but I looked up the maker's website and let me tell you--the workmanship on these is well worth his normal rate.
These are by Pepe's Possibles, and they are truly works of art. They're hand stitched out of durable leather, lined in plain cloth, and the opening is bound. I own the 12" size and (I believe) Samantha snatched up the 18". I love the rich color of these, and they're such a convenient piece of luggage. Even my small one will fit a surprising amount...I can fit a small pair of shoes in there, even! Right now it's got my current sewing projects, my pinball embroidery, and some extra fabric in there with room to spare. The handles (which are sewn on with little heart embellishments--so sweet!) are the perfect size for three fingers, which, in my opinion, is just about perfect.
A little googling will turn up several places to buy such a piece of luggage (try terms like "18th century portmanteau" and "portmanteau luggage"), and they range in price from just under $100 to over $500. Pepe's Possibles seems to be on the lower end of the price range (though again, awesome workmanship!), and another affordable option would be Choochcat on Etsy. I obviously can't speak for customer service, time frame, etc. for either of these vendors since I bought mine secondhand, but they look like good options to me if you've decided you just have to have your own portmanteau. Which you should. Because they're awesome!