As far as I am aware, there are no solo Trumpets or Solo Violins included with the free content in Logic Pro X
You will need to purchase a 3rd party sample set to obtain these...
NI's Kontakt provides for a solo trumpet and solo violin in its large library... but if you wanted sometyhing of a higher quality.. then any of the miriad of Orchestral/Strings/Brass samples sets avilable for Kontakt, would be a good place to start.. (Plus having Kontakt would also open up a world of other 3rd party samples for you to use) However, the majoity of these sample sets are not cheap... though some are of very high quality indeed.
Or.. for a cheaper alternative...
Garritan's Personal Orchestra 4 is a reasonable set to start with... Not great..but Okay... and has a solo violin and trumpet as part of it's library.
There are many other choices.. The above are just two that come to mind..