Note that I don't mean you should start handing out invoices or demanding people purchase various goods and if no one has what you're looking for than you just need to go without. Expecting players to pony up the funds for items you want isn't fair either.
Oh no, what I'm talking about are using communication and gratitude to ensure that players have the opportunity to assist with game immersion and are rewarded for doing so. If you need something for your game and you have a steady membership, ask if anyone either has the prop or has ideas for how you might cobble together a basic version of the prop. If no one responds, that's fine. If they do, fantastic. Thank them and take their ideas on board.
This is a really good idea for small things that may still be costly to have to keep providing. If you need something to represent herbs, seeing if anyone has a lavender bush that they're willing to cut bundles off is much more cost-effective than going off and purchasing lavender yourself. If a toy version of the item can work, such as a stethoscope, you could always put out a call out for that as well in case people have the item in their toy chests.
Sometimes players may be able to loan you larger items as well so don't fear asking if someone has a portable barbeque, trestle tables, table cloths or Christmas trees that they don't mind bringing to the game for a night. You'd be surprised what people have knocking around in their sheds and if you have the storage space, you might find they're willing to literally give you extra chairs, tables and the like. Many players want to invest and feel like they're a real part of the LARP's success and will go to great lengths to help the LARP if you give them the chance.
When you're running a LARP campaign, I'd also recommend being flexible and inventive with the items they bring. In other words, if a player is super excited about making an edible jelly heart, find a way to include that in the game. Don't make the session rely on it if they haven't done it before as no one needs that pressure, but consider it for either atmosphere or a standalone puzzle. If you keep saying "no" or being uncertain about offerings, your players will stop making those offerings as they may start to feel like the offerings themselves are unwelcome and that's not a good place to be.
You want to get the players used to thinking in terms of how they can pitch in with you, give them a sense of ownership of the game and you want to help them get their creative juices going. If they're only ever responding to lists, they'll either switch off or even just forget to mention their own creativity abilities. Plus it can be a really good way to get your own inventiveness going when a player brings something nifty to your attention. You'll be surprised by what you can come up with.
Where people donate or provide items for game, you should certainly make a point to thank them by name on occasion. It doesn't have to be a public thanks every time, especially if the player doesn't like such public commendations, but certainly show your gratitude on behalf of the game. This is far more important to most players than any experience point burst and I have had success in my game going by this alone. This isn't to say that you can't reward people with experience points as it's a nice and tangible way to say thank you, but you shouldn't rely on it. At the very least, a more public reward makes the other players realise that it's an option and helps create an "All In" mentality where the players themselves feel like partial owners of the game rather than passive consumers and customers.
And this last point is equally important. As a LARP organiser, you are almost certainly a volunteer and while their assistance is helping you realise your vision, your vision is meant to be a framework within which they can design their own characters arcs and play their own paths. Sure, feel sincerely thankful for being part of such a great community full of wonderful players that want to pitch in, BUT don't take on a humble, submissive, I'm-Not-Worthy mentality unless you want a quick road to resentment, bitterness and burn out.
It's hard to feel good about something when you sink unpaid hours into it and yet expect yourself to beg for scraps of player assistance. It's a destructive and unhealthy habit encouraged by too many GMs online to combat the old school and equally unhealthy God GM arrogance. You are a volunteer. It is good to ask for help. It is good to delegate. Don't be mean about it, respect your players limitations in time, money and investment, and you'll be fine. No one likes an unnecessary martyr. Breathe and ask!
Building a LARP community around your games where everyone is welcome and able to contribute is a far better model than using a customer service model where volunteers must sink in hours of service provision at the beck and call of consumers. But that's a whole other tangent worthy of an article of it's own.