Dress Huck in old dress pants, cut off at mid-calf. Sew patches to the knees and seat. Add an old straw hat, white shirt and one suspender. Dip the whole outfit in brown or gray dye to make it dingier. The character playing Huck shouldn't wear shoes.
Both Pap and Jim will need a pair of old dress pants with patch on the knees, scuffed and banged-up shoes or boots and suspenders. Add an old, ragged coat to Pap's costume and a vest to Jim's. Wet down an old cowboy hat, and shape it into something that looks like it been around. Cut a couple of nicks in it. Jim could also wear a similar hat or a straw hat like Huck.
For the King and the Duke, think faded finery. Go to the thrift store, buy a couple of old suits and alter them with a little trim and a new collar. You can get spats and a couple of cheap top hats or bowlers at a costume shop. Bang up the hats a bit. Make 18th-century ties with thin black ribbons.
The women will need a long dress or skirt and blouse in cotton or cotton polyester. The bodice or blouse should have a high neck with long, full sleeves that taper to the wrist. Add a high-waisted fabric belt and bib apron. If you don't have a petticoat, use a long full skirt in red or white as a slip under the dress. Decorate a small straw or flat brim hat with flowers, and finish the costume with dark stockings and high-top boots. You may also find easy to use patterns for bonnets.
Other male characters, such a Tom or townspeople, would wear newer, cleaner versions of Huck, Pap and Jim's costume. They will need dress pants with suspenders over white shirts, cleaner boots and wide-brimmed hats. The adult men will need coats or vest and black ties depending on their social status.