One key to building successful boosters is to find the few people in town that have decent incomes and tap them. You can't keep asking families without much liquidity to donate, but this tends to be the "go to" option for EVERY organization, so the "haves" get tapped again and again to contribute.
The other option is to crowdsource everything - find ways that everyone can chip in just a little bit, so no one pays a disproportionate amount.
There are other ways, though.
A third possibility is to expand beyond the town - extended family, local business, DonorsChoose, and the like - but that requires significantly more legwork, and your boosters aren't set up for that. Yet.
And a fourth possibility is for your Boosters to train someone to write and apply for grants - and have that be their sole job as a Booster. You never know - there's always money to be found.
So you need to get a parent of a 7th or 8th grade band kid and get them 100% on board with your vision. Push them into leadership on the Boosters and have them start networking and bringing in others. You'll likely only have 5-6 families involved in the Boosters at any one time because people simply don't have time to support groups like that. Have them contact what local businesses there are. Contact the company you guys pay the electric bill toward. Contact the gas company. Lots of those companies are aching for goodwill right now, after the Freeze this year. Have them talk to any and everyone they can to support the Boosters.
Building your boosters will mean you have a half dozen dedicated parents that can coordinate and work with other people involved in the community - but you're likely never going to get "band moms" and "band dads" in a small or working-class towns the way you do in the suburbs. There's just too much to be done on a day to day basis for parents to relish the idea of signing up for MORE work, unpaid, at the end of the day.