Where can I find an agency that helps making my apartment accessible, hopefully through a grant?

Try a local chapter of habitat for humanity. Occasionally some locations that don't have a new home being built may take on this type of project. You might also try the Eastern Paralyzed Veterans Assoc. I've heard that they occasionally give direction to non vets. Are you affiliated with a local house of worship? Perhaps they can help. What about Catholic Charities or some of the Jewish Charitable organizations. (Both of which help folks of all beliefs). They might have some ideas. Hope this gets you started and if an agency says they don't provide that help, ask them if they know who does!

Where do you live, in the United States? A lot of communities have lending closets in their senior centers for equipment to help with toileting. I'm not sure I understand the heating pipe and how it blocks your front door? What are you wanting to add to your toilet? My mom used a raised toilet seat on a normal toilet (about $20 to purchase) and some rails (about $35 to purchase). Are you wanting more than that?

And what are you wanting for your bathtub? My mom had a cut-out in her regular tub that you can buy a kit for, for about $400. It would cost to have it installed. Then you can put a shower stool in the tub and sit on it. I'm not aware of any agencies that fund in-home help like you're seeking.