Hi John,  A few years ago I had the same question.   Actually, buying
a ready-made cart can cost a lot of $$$, so I looked on eBay until I
found a reasonably priced antique carriage with 13-inch wooden spoked
wheels.  I asked the seller to remove the wicker basket portion and toss
it as I wanted only the frame.  I bought it for $50.00 (a great buy)!

It was too big to ship from St. Louis to my home in Long Island so I
asked the seller to saw off the handle a few inches from where it was
attached to the base frame.  They wouldn't do that until after I paid
them.

When I finally got the carriage ($28.00 shipping) I took it to a local
welder who put the handle back on perfectly ($20.00).  I then took the
carriage to a firm that does sand blasting and powder coating; this
cost me $90.00.  The color I choose is a crinkled copper -- looks great.
He lightly sand blasted the wooden wheels and I stained the oak spokes
with dark walnut Minwax. (He could not powder coat them because they
are wood and the heat necessary for powder coating would burn them up).
So now I had a beautiful carriage frame.

I went to a local carpenter with my 20-note Jaeger & Brommer crank
organ and asked him to make me a box that I could attach to the
carriage frame and put my organ at a comfortable height for cranking.
I left the back of the box open so I could store my rolls, etc., in
there.  This box cost me $150.00.  Total cost $338.00.

Hope this gives you the info you asked for.  It's a nice summer project
for you.

Bill Sharkey