Olivia and Ava are lovely and sound very sweet together. I don't think it matters how popular they are as long as you love them.
Other names ending in a:
Amelia, Amalia, Annabella, Arabella, Alicia, Alisha, Alyssa, Alexa, Alexia, Alexandra, Alexandria, Amanda, Aurora, Aurelia, Angela, Angelina, Angelica, Anita, Anika, Anoushka, Anastasia, Araminta, Alma, Alba, Amna, Aqsa, Arwa, Asma, Agatha, Asia, Asha, Andrea, Ada, Atlanta, Anya, Ayla, Aria, Amina, Amira, Amara, Alara, Aliza, Alina, Aniya, Anaya, Amaya, Ariana, Adriana, Antonia, Aliya, Brianna, Bettania, Bianca, Bella, Barbara, Brenda, Ciara, Clara, Carla, Cara, Carolina, Cora, Cassandra, Clarissa, Cordelia, Candida, Calista, Cynthia, Claudia, Celia, Cecilia, Cassia, Catriona, Christina, Dana, Delia, Dorothea, Dakota, Deanna, Daria, Diana, Daniella, Emma, Eva, Ella, Elena, Elianna, Elina, Elizabeta, Eliza, Elsa, Erica, Etta, Enya, Eloisa, Evangelina, Emmanuella, Francesca, Flora, Fiona, Fionnhuala, Freya, Fenella, Fabia, Felicia, Gabriella, Giovanna, Gloria, Gemma, Gina, Gia, Gianna, Georgia, Georgina, Hanna, Halima, Hera, Heda, Isabella, Isla, Ilsa, Iona, India, Ida, Irena, Imelda, Isadora, Inaya, Iqra, Jessica, Joanna, Julia, Julianna, Jayla, Jada, Jemima, Jenna, Jerica, Jocasta, Kayla, Katya, Katerina, Karina, Kia, Kaia, Kya, Kyra, Kyla, Kendra, Keira, Lilia, Liliana, Lila, Lina, Lara, Lana, Laura, Lorna, Lisa, Liza, Livia, Layla, Lyla, Lyra, Luna, Lola, Laiba, Lea, Lydia, Linda, Latasha, Latoya, Larissa, Leona, Leandra, Lavinia, Louisa, Louella, Maria, Martha, Mia, Maia, Maya, Mila, Mina, Mira, Myra, Mika, Malaika, Melissa, Marissa, Miranda, Matilda, McKenna, Mariana, Marina, Monica, Mala, Marwa, Nia, Nina, Nadia, Natalia, Natasha, Nola, Nora, Orla, Ophelia, Octavia, Pippa, Philippa, Patricia, Priya, Pandora, Paloma, Rebecca, Rosa, Rita, Rhianna, Roksana, Rhea, Rowena, Ramona, Sara, Sophia, Samantha, Stefania, Skyla, Savanna, Sienna, Sierra, Stella, Sasha, Sylvia, Suzanna, Selina, Serena, Sabrina, Saskia, Soraya, Shania, Shayla, Shauna, Shona, Sorcha, Sadia, Syeda, Safa, Tia, Tianna, Thea, Tyra, Toria, Tessa, Tara, Tamara, Tayla, Tabitha, Teresa, Tina, Ursula, Una, Ulrika, Victoria, Vanessa, Viola, Valeria, Veronica, Virginia, Valentina, Willa, Wanda, Xenia, Yusra, Zara, Zia, Zoya.