I’m presuming you mean the a frame cross member, If I remember correctly from a few other threads you can’t buy that particular part anywhere and most people repaired it which is what I did on my 110 and my 90 or fabricated there own I’ve never seen one for sale but it’s been a while so someone might be selling them now . I place to try is Replacment chassis parts there on Facebook and eBay they might be able to help .
https://www.paddockspares.com/parts...fender-crossmembers-to-the-end-of-300tdi.html
The bit you have circled is the rear cross member. It's one of the first bits to rot out on the chassis and there are plenty of repair sections available. The main hassle is taking the body off, but you've already done that.