For both beef and dairy bulls (semen), I primarily deal with Select Sires. Not just because I worked for them before, SELECT semen (genetics and quality) are better than other companies. Then good people are working for them. It is plainly true that Select bulls are producing more carcasses for Certified Angus than any other competitors.
However, if you buy semen from other companies and want me to use them, I have no problem with that. Or if you have particular bulls collected, that will be a reasonable way too. Either way, I will help you choose & buy right genetics for your desire and herd. Then I simply breed your cows with my best (that I believe it most important!!).
I can work with any breeds but probably not with buffaloes.........
*I'm not affiliated with any semen or other industrial organizations, nor involved with particular breed organizations either.