kitty, I see you are in Germany and My bank account is in the US so probably zero of what I can tell you is wroth anything to you but anyway. . .
There are (accounting or financial) programs out there which will allow you to print your own checks.
I started printing my own checks in the good old days of DOS with a nine pin dox matrix printer. I copied the layout of a preprinted check as close as I could (it took many hours but I dodn’t have a life then) but it was still very obvious the check was home printed. The special digits at the bottom, I designed the font myself.
When I upgraded to Windows and an inkjet printer I had to do all the work over again. In MS-Word97 I designed the check and I needed the special bank font. I found a sample on the Net and a program that allowed you to design your own fonts and so I built the font. (As you can see I still did not have a life).
So, I can print blank checks or print them already filled out which I like better. I also designed a background pattern which I print in very light blue and is a repetition of my name in tiny print. It makes the check look almost “real”.
Now, if I had some dollars in the bank my checks might be worth something.