A couple of decades ago there was a nifty little gadget on the market for ham radio operators called the "Datong Morse Tutor". It generated random 5-character groups of all letters or mixed letters/figures, at speeds from a few words per minute to in excess of 30 wpm.
I used it to get my marine commercial operator's speed up to exam standard. It worked well. Look in the ham radio magazines for second-hand units, or find your local ham radio club and ask if anyone has one or knows someone who has and who would be willing to part with it (and almost all of them will)