Most gardeners are born scroungers or eventually learn how find what they need for cheap. There are a lot of free or nearly free materials out there that, with a little effort, can be turned into something of use for the garden. Keep your eyes and mind open, and let everyone know what you are looking for. Put up a garden wish list on your bulletin board, send the list out to garden listserves and support groups. You never know what people have lying around!
It doesn't have to be bought in bags. Check local stables, including the police if you are in an urban area. Make sure the manure has been composting for a sufficient amount of time.
Most municipalities now collect leaves in clear plastic backs for their own composting programs. Either beat them to it on collection days or order their finished product.
Rake it up yourself, raid neighbours' curbside collection bags, but beware of herbicide-treated lawns.
Power companies, tree service companies and municipalities chip their trimmings, usually on right on site. Also talk to carpentry/woodshops.
Consider using coffee grounds from your local cafe, spent hops from your local brewery, monument companies for granite dust (a potassium source), feed mills for corncobs, farmers' spoiled hay and straw, construction companies for straw and topsoil etc.
You can often find old pallets (great for making compost bins) lying around in dumpsters at lumberyards and construction sites, as well as wooden packing crates (often perfect planters, just as they are). Just make sure that the wood isn't pressure treated.
Pipes for posts or trellises can often be found in dumpsters at construction sites. Also plumbing companies will often throw out damaged or small pieces of PVC (plastic) pipe.
Scrap wood from various sources (see above), used snow fence (sometimes free from fence companies who rent it to construction companies).
These come in handy for watering, container gardening, hauling anything and everything, protecting newly transplanted seedlings, mixing ingredients. Can be found at restaurants, construction sites, dumpsters etc.
Check with large scale juice or beverage companies, post on your local freecycle website.
Many nurseries, garden centres, seed companies, and Parks Departments will give away seeds and annual plants at the end of the planting season (usually around mid-June). Also consider saving your seeds to use and trade the following year!
Keep your eyes peeled at garage sales, auctions, second hand stores. Ask around for broken or forgotten tools and see if your city has a tool lending library.
Adapted from Food Share's, Resources: How to Find What You Need http://www.foodshare.net/toolbox_month04.htm