Yes I fully understand what your saying.
I live 200 miles away from my mother-in-law so not easy to help that much but thought I would just see if I could get this to work.

Wrong time of year to contact most sports celebrities aswell so not ideal

If it's going to be "hard," then you probably shouldn't do it and find some other way. You want to be taken seriously, you have to have a professional attitude - hence the letterhead request. They're not going to give out stuff on your say-so in 140 characters or less. Some celebrities have foundations they work through for charitable requests.

As well, considering it's almost mid-June, and your fundraiser is in August, that's not a lot of time for celebrities to get a letter and respond.

It could be hard to get someone from the charity to actually sign a lot of letters to send out to celebrities.
I would probably want to send out about 50 letters I guess

Twitter won't do. They don't know you from anybody else off the street. You'll need a formal letter, on the organization's letterhead, to the celebrity explaining what the event is and what you're looking for. Preferably, too, signed by an official from that organization, not you. Otherwise your request won't be taken seriously and wind up in the garbage.

Afternoon all,

I am wondering if anyone can give me any tips, here is the situation.....

My mother-in-law is a care worker supporting people suffering with alzheimer's and dementia.
She is walking the great wall of China in October and has a target of £10,000.

We have organised a charity ball in August and am hoping to get hold of any signed photos or merchandise than we can auction at the ball so all money raised can go to the alzheimer's society.

I like in the UK and was wondering if any of you would have any tips or suggestions of who to contact that may have the best chance of sending me anything.
I am happy to email or write letters to send in the post.

I have been messaging hundreds of celebrities on twitter asking if there is anyone way can help but not had one reply.

Its all for a good cause and I know the hard work the carers put in to give these people the best possible live they can have.

Please message on here or feel free to direct message me, any help much appreciated.

Thanks
Mat