First of all, I purchase zero biomass. Gee, it's free for the taking. Yes, just my labor and time. Maybe lots of labor. Where? I look around. Neighbors sometimes have leaf and brush piles just begging to be taken away. Local landscapers love to unload in my yard rather than pay to dump at the landfill. Professional woodchippers also wish to avoid dump fees. Utility companies often clean their easements and are willing to dump the debris someplace. Sometimes they have it piled in their base yard where one can load it into a vehicle and take it home.
Check with your parks division. Back in NJ we were able to pick up truckloads of tree branches and trimmings, which we cut up into firewood. Every fall the towns collected leaves, so people set out bagged leaves on the curbs. So every day coming home from work I picked up a carload. Plus there was a small woodmill not too far away. Sawdust and scrap was free for the loading. Cabinetmaking shops often have sawdust and scrap they wish to have people cart away. I wish these things existed near to where I now live, but alas, no. So I have to look for other sources.
I didn't have much of a lawn in NJ, but I never lacked for grass clippings. I just put up a notice on the feed store bulletin board that I would pick up bagged clippings within town limits. I developed a steady pick up route that way. I would run my round that same time every week so that people knew when to expect me. Bags were ready out on the curb. But here in Hawaii I have lots of neighbors with large expanses of grass. So with my trusty lawn tractor, I can mow to my hearts desire and collect scads of grass clippings.
Here on my current farm I have plenty of vegetation. Brush, grass, weeds, excess trees, bamboo, bananas all give me plenty. It just takes my time and labor to collect it. But I still do some gathering looking for particular biomass. Abandoned wild fruits do great for feeding the garden plants via trench composting. The taro responds to it especially well. A feral grass called guinea grass is rather easy to harvest and is a good filler for the pallet grow boxes. And I've discovered some side roads that have lots of partially rotted tree branches that are great for the foundations of hugelkultur beds.
Cardboard and newspaper can be collected from friends, or put up a notice that you are looking for it for the garden. Lots of people would like to see it used instead of just thrown in the trash. Stores often have cardboard they will let you have. Wal-Mart has it already bound, ready for the back of your pick up truck.
If you are lucky to live near a vegetable farm, then seek out the farmer. There will often be a packing shed where reject veggies are thrown aside into a bin. Ask if you can load up some for your compost pile. Or ask if you can glean a harvested field so that the broken rejects can make your compost.
At the store level you might not have any luck when it comes to discarded vegetables, because pig and chicken owners are competing for that. But it doesn't hurt to ask.
If there are horses in your area, stables can be a source of waste bedding and manure. One time I cleaned out an old barn for an owner. Quite a bit of labor. But part of the deal was that I'd have first dibs on the manure for the rest of the time. So for 17, or was it 19, years I got all the horse manure I ever wanted.
Cleaning manure out of pastures is work and takes time, but if that's your only source for manure, it's worth taking the time. I know of a young girl who made a little business cleaning up horse owners' corrals. She then sold the manure at a flea market. Good for her! Way to go!
Starbucks and other coffee shops will give away coffee grinds. Just supply them with a lidded 5 gallon bucket and be sure to pick up daily.
Earning money by mowing somebody's lawn will also give you the lawn clippings. Perhaps a senior citizen neighbor, something close so that it's convenient. In the fall, raking leaves for neighbors accomplishes the same.Tthe extra money can come in handy and it cones with a biomass bonus.
Of course you could always fall back on buying biomass. Straw is the number one thing to come to mind. But don't rule out spoiled hay. If there is a feed store in your area, put up a notice. Local hay producers and farmers sometimes lose a load of hay to moisture and mold. Yes, it will have seeds compared to clean straw, but composting it should take care of most of the viable seeds.
Over the years I've used all these ideas at one time or another. Landscaping businesses were my best suppliers.
I use a LOT of biomass. Anytime I find myself with surplus, I cobble together another growing container, fill it up, and plant something. But I seldom have a surplus bonus because I'm constantly renewing the beds and containers I now use. Plus I use a lot of mulch.