I did it myself last summer. Depends on your tank size but I spent $250 on resin and a funnel. This now displays unavailable but there’s are other similar types by this seller:
Water Softening Resin Upgraded 10% Cross Linked 100% Ion Exchange High Capacity Replacement Softener Resin - Cubic Ft (1.0 Cubic Ft) https://www.amazon.com/dp/B07PJ8RM98/ref=cm_sw_r_cp_api_glt_fabc_YF019NS8MYVDD8MFD0AV?_encoding=UTF8&psc=1
All in all I needed 1.5 cu ft of resin.
That said, the process was conceptually simple. But it was extremely messy, dirty, slippery, and it sucked. I did save $1400 by rebuilding my softener rather than getting a new one. Many installers will no longer do a resin bed rebuild because the process is so time consuming and messy. But if you could find someone to do it for $100-200 in labor, definitely take advantage of that. I would happily pay that amount.
Here is the video I followed by Gary the water guy: https://youtu.be/Z47JGH1l39o
The resin is inert, but not biodegradable. So I’d avoid letting it run into the storm drain or into your plants. They need to be disposed of as waste. The messy part is when you try to wash it out of your tank while trying to catch all the little beads in a canvas or woven bag. It’s reallyyyy slippery.