Fund raising for a shelter (or any organization for that matter) is always a challenge. It is time comsuming and can be very frustrating. It's also a matter of setting priorities. Setting the priority on fund raising diverts resources (mostly personnel) from the daily operations of the shelter, but not diverting those resources creates long term hardships through lack of funds. Shelters are usually best served by approaching the people in the community they serve.
Petco/Petsmart and other companies do provide some support in the form of adoption support and space to display a few animals. In my experience they also will usually allow you to put up a series of photos and info of available animals, but someone will have to take the time to take pictures and keep this information up to date. You might also approach them about donating short dated or expired food. They can't sell it, but it's perfectly OK for the animals.
Some supermarkets will allow a bin to be placed in the exit for donations of food/toys/treats. This also involves someone committing to regularly check the bins and bring the donations to the shelter (and there is an investment involved in purchasing the bins in the first place.)
Adopt-a-cage (make a donation and get a plaque on a cage for a year) or other visible recognition of donations helps too. Publicity about the program will be necessary. Highly visible community events such as dog washes or a dog walk as fund raisers can be very helpful in both bringing in funds directly and creating public interest in your organization.
Talk to the local papers. Ours will run a weekly picture of a "pet of the week" and also a quarterly half page (sponsored by local businesses) which features pictures of a number of available animals. These both prove to have high adoption rates for the animals featured and also provide general interest in the shelter. They will also donate a set amount of free advertising for other events.
Talk to the local cable provider. Most areas have a cable access channel and will provide a low cost (or maybe even free) half hour slot on a weekly basis to have a little show. Ours features available animals and each episode features some sort of educational topic. This requires quite a time commitment and some video recording and editing skill and resources.
Approach local businesses. Many will sponsor a cage, event, or other specific item. Avoid asking for funding for ongoing operating expenses since this has a high probability of rejection. Something that the business can put a plaque or display their name in (such as a brochure) usually allows the company to write off the expense as advertising (check with a tax professional to be sure about the laws in your area.) The laws are such that small businesses are usually not able to write off donations to charatible organizations.
If the shelter receives local county or city funding an increase in profile helps when approaching the officials about increasing the official budget support. It helps to have good records of what it takes in personnel and supplies and materials to clean cages and feed and care for the animals. Careful work load studies, accurate accounting of food and material usage, and an accurate and detailed budget are important tools to have at your disposal before approaching these officials.
The important thing is someone (or a group of people) must commit the large amount of time needed to organize, talk with businesses, design and print brochures, take pictures, take animals to adoption events, pick up donations, provide educational talks to the public, etc. It may not be as rewarding as working directly with the animals, but it is just as important. Without funds you won't be able to buy food, medicine, pay the lease or electricity, repair cages or equipment, pay salaries, etc.