I think that when I travelled to Namibia I got my injections about 6 weeks in advance but they last between 1 and 10 years (depending on what you get) so you could get them sooner. I think the tropical medical place have a website.
Yes they're an awful price, I have to get some for Bali and I'm toying with the idea of not bothering, but then thinking of the consequences of getting sick over there!
I looked into this a couple of years ago. This place was way cheaper than anywhere else. At the time it struck me that the thinking was that if you could afford an exotic holiday then you could afford to pay over the top for your vaccinations.
Airport Vaccination Centre
Medical Centre
Dublin Airport
In england there is the tropical disease centre and I think it has a good website or it might be under the british forgein office. Sorry no link, try google.
For malaria, if you get the daily tablets, you need to take them 1-2 weeks in advance, during hols and 1-2 weeks upon return.