So, we are a Halo costume and prop making community whose primary focus is teaching others how to build and wear their own Halo Universe costumes. We do partner with MS, Halo Studios, Conventions, and Charities for appearances, but as an Organization we do not sell or rent our costumes and props.
Our members make their own costumes, so for one thing the costume is going to scaled, sized, and fitted to the creator and most times won't be easily or comfortably worn by someone else. Also, as they are someone's personal creation that they have sunk blood, sweat, and tears into, most members are not comfortable with someone wearing their costume, especially out of their supervision.
That said, our members do volunteer their time to make appearances, at no charge, for events. It is possible we could find a member or members in or near the San Francisco area who could work with you and the Hospital* to make an appearance.
SubXzeroXhero would be the person in our Pacific Regiment who would be the person to talk to about organizing a potential event.
*As someone who has participated in and organized Children's Hospital Appearances for this and other Costuming Communities in the past, you really need the Hospital to be on board and approve of all Character appearances. Sometimes they have special requirements in regards to disinfecting the costumes, limits on materials, and approvals from thier security team.