Any jeweler has does custom work should be able to cast german silver. About half of them will have equipment large enough for it. The casting is not very expensive, but if you want to make an investment cast copy of something you need to make an RTV or rubber mold of it. That stuff is expensive and requires specialized equipment. I would guess about $500 to $800 to get it done. Good news is that once the mold is made you can put it into production easily. Likely fit 5 or 6 of these in a single flask. So it would cost about the same to make six as it would to make one. Keep one and sell the rest.
Problem is that trigger guard is not very thick. The mold will make an exact duplicate, but you get a small amount of shrinkage in the casting. Then it gets even smaller after finishing. You could probably get a good one with an investment cast. Sand casting is a lot cheaper. They just use the original to make the casting mold. But that makes an impression that once again matches the original. Problem with sand casting is that it requires a lot more finish work than investment cast. You would end up with a trigger guard that is way to thin. Sand cast is not going to work.
What I would recommend is carving a wax model.
http://www.freemanwax.com/ferris-file-a-wax-carving-milling-waxes.html You would want the blue one. Some dealers have blocks any size you want, but it is probably easier to just get a couple one pound blocks and weld them together. Welding carving wax is pretty easy. Just use a hot butter knife. Get a block of wax and carve the trigger guard how you want it. You are not going to get an exact match carving by hand, but can get something as close as you can make it. If it is a flop, just melt melt down into a block and start over.
Nice thing with carving wax is that it is what is used for lost wax investment casting. It is a direct cast. That cuts out the most expensive part. It is very easy to carve. It can also be used for a sand cast mold. Find a foundry before carving. Most big cities will have at least one. But you can always mail in the model. Email photos of the trigger guard then call and talk options for casting if you were to send in a finished wax model. Decide if you want to go with investment cast or sand cast. Foundry can help you decide on that. Need to know that before carving because once again, you lose metal in the finishing. If you carve the wax slightly larger than what you need, it will will finish off at the size of the original. Sand cast requires more finishing work so it need to be carved thicker than one for investment cast.
If you send the original to a foundry they can male a wax model for you and send back a casting that will match it very nicely. Foundry can do a 3d scan, enlarge it just the right amount, then make a model with 3d printer. $$$$$$$$ If you want 500 of them, that is the way to go. Probably best to just carve it yourself. Tell the foundry to just cast it and send it back unfinished. Just be away that wax models are destroyed in the casting process.
If you carve a wax model and have the foundry cast it, it will be expensive. But not unreasonable. Track of the Wolf has molds on hand and has the foundry cast a whole mess of them at once. A one-off is always going to cost more. Hard to say exactly what the cost would be. It costs a certain amount for money for a foundry to make a mold and do a pour. But they can do many pieces in a single mold. If they are already doing a german silver cast it is easy for them to add it in. If they are casting 100 pieces all at once and you only have 1 of them, you get to pay 1% of the cost. If it is something that do not do very often then you either wait until they do a run of german silver or you pay for the mold and pour all on your own. Local foundry is more convenient but if they don't do much with german silver it is going to be expensive. You can save a lot by looking around for a foundry that does lots of german silver.
