Where is the first to second shift accumulator located

I had the transmission rebuilt a couple months back. Due to a bad transmission. Now the transmission for the most part does fine. But every so often it will shift hard in first to second gear. All other gears are smooth. When I stop at a store shop for a few and get back in the car it's completely fine again. And can go days sometimes even weeks without rough shifting. No codes pop up. The guy who rebuilt the transmission says it sounds like it could be first to second shift accumulator. Is this hard to change? Where is it located? Do I have to replace the whole valve body? I keep looking up the first to second shift accumulator for my car and solenoids just pop up.