You might find that your grandparents have got one in the attic anyway. There are many good ukulele makers all over the world. A guitar maker might make one for you, though a small guitar does not sound the same as a ukulele; the construction methods and materials are different. Members of the Ukulele Orchestra have obtained good ukuleles from: Marshall Stapleton, Pete Howlett and many other manufacturers including Hawaiian experts, but a relatively good one can be obtained from most music shops. The traditional wood for ukulele construction in Hawaii is Koa, though many ukuleles are made from other woods. In the UK, a reasonable uke can be found for around 25 UK pounds although one can find old collectable ukes or high quality instruments which cost thousands of pounds.

Ukes are tuned g' c' e' a' for soprano, and d g b e' for baritone.

The easiest thing is to write for soprano (or concert or tenor: they all usually have the same tuning) in treble clef (assuming 12 frets on the instrument this gives a range from middle c upwards to a, an 8va and a 6th above).

For baritone, write as though its a classical guitar with the two lowest strings removed (so that means assuming 12 frets, a range from d below middle c, up to e on the top space of the treble clef). Your players might have more frets available and if so could go higher.

This means the baritone is a transposing instrument, for example, you write a "d" below the bottom line of the treble clef, and when the baritone reads and plays this, it sounds an octave lower than notated. As with guitar.

"Normal" ukes are tuned in re-entrant fashion (g' c' e' a', rather than g c' e' a'). This enables "between the strings" melodic work (campanile), sustaining adjacently pitched notes on different strings, for a bell like effect. Its a bit idiomatic for some tastes so you might persuade your players to use a low g string (g below middle c, instead of g above middle c) which will give you a wider range on the soprano uke. Baritones are rarely tuned in re-entrant style.