@RuthJennaway
First, it should be noted that PowerApps for Office 365 is included with all the Office 365 plans. So, there is no price involved for PowerApps license. If you need to upgrade beyond that license to a P1 or P2 license, then...well, I've never seen specific pricing for those plans for non-profits. But, I don't believe, based on your statement, that you need anything other than the standard plan.
Now...as far as Office 365 for non-profits - Yes, this is very much available. The current plan for non-profits is an enerprise plan (E plans) or Business plan. Non-profits pay about $4.50 per month per user for E3 licenses and nothing for E1 licenses or $3 a month per user for business premium and nothing for essentials. You can check out non-profit plans at the Office 365 nonprofit site.
All of the above include PowerApps of Office 365.
Also, you might want to check out TechSoup for more tech savings for non-profit organizations.
If the orgnization is an officially registered nonprofit, the process of getting Office 365 for nonprofit is pretty simple. All the eligibility information is on that site.
I hope this is helpful for you.