Violin solos are plentiful! Some of my favorites are the Sibelius violin concerto: https://www.youtube.com/watch?v=ITTbY1n3Iz8, the one by Bloch: https://www.youtube.com/watch?v=aTEnPximays, and the Tchaikovsky: https://www.youtube.com/watch?v=cbJZeNlrYKg. Feel free to check out Mendelssohn's and Beethoven's if you're looking for a more classical feel.
For viola... good luck! There isn't a whole lot for viola + orchestra in the repertoire. The Stamitz (https://www.youtube.com/watch?v=AaHRWcEfroU) is popular among violists but it has that purely classical sound. Harold in Italy by Berlioz (https://www.youtube.com/watch?v=Q4NC4E5RXik) is about the only great Romantic-sounding orchestral piece featuring viola out there... unfortunately, this piece isn't very virtuosic. Hindemith wrote Der Schwanendreher (https://www.youtube.com/watch?v=NkFbrUEhSCc&t=7s)—not my favorite but I guess it's worth mentioning.
There is, of course, another viola concerto written by someone you might know. 😉 It hasn't been performed yet but it was composed by a violist! (https://www.youtube.com/watch?v=gYW8K6N4FEE)