Where do i get larger injectors for my 1900 ford Mustang?

Off what vehicle could I swap bigger injectors from.

The stock 19lb injector is good up to 304 hp at 40psi.24lb is good to 384 hp and a 30lb is good to 460 hp. The link below is from a popular Mustang forum and its a list of parts you can get off of cars at the junkyard.Scroll down until you get to the injector list where it shows make, model, and injector size of each particular vehicle.
http://forums.corral.net/forums/5-0-5-8-engine-tech/1092402-junkyard-horsepower-summary.html

I don't think they were using fuel injection in 1900.

I think it is the pump you should look at changing.

I really don't recommend using used fuel injectors. At any rate adding larger fuel injectors when there's no need for them, will yield no improvement at all. To warrant larger injectors you're going to need to have done substantial internal modifications and/or added forced induction.

If you up the injector size, the MASS air meter needs to be updated also.