[BR3W CITY]

You can find an ECM on ebay or in a junkyard, as long as its a v8/auto donor, and is ROUGHLY the same year as the engine (makes a bigger difference with dbw). Your tuner should be able to set it up to work with your engine just fine, and the 4l60e should already be the trans its setup for since 80% if 5.3 vehicles came with em.

I bought a complete harness, with its own relay box and fuse box, of very nice quality setup for my swap, for about $600. The used ecm should be like $65, and tuning can be anywhere from $150 (removing vats and setting up the year/engine combo) to 400 (dyno tune, tuning for cam etc)