The torah forbids creating artifacts used in the beis hamikdash (based on the verse lo taasun iti (Exodus 20:19), see Talmud Avoda Zara 43a, Rambam Beit Habechira 7:10 and Shulchan Aruch YD 141:8). This would seem to include the trumpets, which the gemara elsewhere learns could be made of any material. Therefore, can one create, buy, sell or own these instruments?
The Minchas Chinuch in Mitzva 254 writes (my own translation):
It seems clear that although the Gemora only specified the design of the building, the design of the utensils the Shulchan and the Menorah, it is not limited to these but rather it is also prohibited to make a likeliness of the Mizbeach Penimi. However, it seems clear that this is specifically those utensils whose form was explained in Tanach or through Ruach Hakodesh, whereas other utensils are not prohibited from replicating. For every day in the Beis Hamikdosh there were 93 utensils of service as the Mishna explains, and there were many utensils such as basins and spoons and other untensils. Would it enter one's mind that it is forbidden to produce replicas of such items!?
The trumpets could very well be included in the latter category of utensils that do not have a specific form. Rambam (Kli Hamikdash 3:5) writes that they must be made from silver and ideally out of a single block but places no constraints on the measurements.