Dream Audio Tools' Repetitive Bass You can layer it with their Indie Fingers Series DAT has a lot of instruments (including mandolin, dobro, bouzouki) that pulse and blend with each other, and with other libraries. They are always inexpensive, and often go on sale. The bass is particularly useful when paired with syncopated rhythms.
EDIT: There is a 50% off sale now on Dream Audio Tools. Use code Easter2019 at checkout.
Other recommendations: Heavyocity's Scoring Guitars, Sonuscore's Origins series, In Session Audio's Fluid Harmonics & Fluid Strike, and Sonokinetic's Ostinato series (strings, brass, and woodwinds).