There are custom mold makers that can fabricate what you want, or that you can purchase made to order. Accurate Molds comes to mind. Tell them the as-cast diameter you want and the alloy you plan to use. They have several options that approximate the original look if that's your intent. I don't think they offer molds with hollow base plugs, as found with original bullets, but MP Molds does.
But, 470Evans is correct in noting that the good old 454424 will often cast a bullet, depending on alloy, that's a good fit and shoots great. The original hollow base bullet is problematic. The bearing surface is all hollow and the weight is well forward in the long nose. If that bullet tips even slightly, which is a huge problem in the stubby MKII cases, your bullets will keyhole badly.
He's also correct in noting that your goal is to find a good fit for your cylinder throats. Yes, bore diameter is important. But, if your bullets are a loose fit in the throats you'll likely never get better than mediocre accuracy and excess leading, even at modest velocities.
Hint: cast on the soft side (to permit obturation) and use (or make your own!! ...ask me how) an old fashioned "sticky" lube.
Enjoy!
