There so many out there, I'll only list the ones I've used recently.

Aim Surplus, DPH Arms, Midwest Armory, Centerfire Systems, Sportsman Guide.

Aim Surplus has kits for $89.99 if you get 2 or more.

DPH Arms has good quality kits at a good price. You can also get US compliant parts, flats, tools & accessories.

Midwest Armory has excellent US parts, they charge a flat rate of $3.85 for shipping and it's free for orders over $50.

Centerfire Systems has cheap underfolder kits with dark bores. Pam offers discounts to AR15.com members

Sportsmans Guide has sales on ammo and mags, cheap, but there usually backordered items.