I am very sorry for your loss. Unfortunately, financial assistance from charities, etc. is very hard to find; they tend to prefer to use their resources on the living.
I have a suggestion, though it can be a controversial one. I do not intend to stir up a debate with this, but the poster needs help, so I'm going to talk about an ugly reality. If anyone is offended for any reason, I apologize in advance.
OP, if you are white, please ask your hospice's social worker about minority-owned funeral homes. It's an odd quirk of the industry that the funeral business is one of the most racially segregated in existence, and most hospital and hospice social workers don't rock the boat when it comes to this situation. They know which businesses serve whiich populations, and they generally will only give you the names that match your demographic, unless you specifically tell them that you want them all. As a general rule, minority-owned funeral homes are much more willing to work with families on payment and on ways to economize on their services. When I had to make arrangements after my brother's unexpected death, I found that they were much more reasonably priced, but the social worker at the VA was VERY reluctant to give those names to a white woman, because she was afraid that I was going to get very insulted. (The staff at the mortuary I used were very gracious, though initially a little surprised to be hearing from me.)
Remember that if your father was a veteran and has predeceased her, your mother may be entitled to be buried with him in a VA cemetary. The VA will not normally conduct a service for a civilian, but you would not have to pay for a plot, a vault, or a headstone.
http://www.cem.va.gov/bbene/eligible.asp
If you are willing to do a direct cremation, contact the crematorium directly, rather than go through a funeral home. In most states crematorium operators are licensed to transport remains. (They may not be listed in the phone book or listed online. Call your state licensing board to find out how to contact them.) Very few funeral homes can do their own cremations; they contract out the work, so you are paying them an additional fee to make the arrangements. Once the cremation has been done, the funeral arrangements can be taken care of directly at your church, and you can transport the cremains yourself. (If you are scattering the ashes, you don't need to buy an urn, either; you can just use a nice box that you purchase anywhere you like.)