If youi find a rifle which is exactly what you want it's usually cheapest to buy already assembled. Preferably used, local, and a private sale so (in most states) you don't pay shipping, tax, or FFL fees. Gun shows are a good place for those, sometimes internet auctions if the seller is in your state.
If you plan to change many parts such as custom triggers, stocks, handguards, even barrels it usually makes more sense just to buy all the parts except a stripped receiver off the internet and get the receiver at a local gunshop. Shipping costs are lower if you get several parts from the same suppler. You only pay tax on about $100 for the receiver.. With that method you get all new parts.