Quite a lot to consider when including a mirror in your plan I would recommend proceeding with some caution.
You should take time to understand what exactly will be reflected in it and what this will do for your design.
It should not be in a position where it can focus the sun's rays onto anything or anybody.
You may want to consider the effect it will have on the birds who may not understand that what they see is a reflection and go crashing into it.
Quite small ones with Gothic style tracery over them, i.e. they look more like a window when in place, may offer something interesting in an enclosed space and be safer. I think large mirrors bring a lot of problems with them.
I have no experience of using acrylic but I think most garden mirrors are made of this anyway.
Got a customer with a plastic mirror on the fence, about 6x4 used to add intrest to a corner, reminds me of a fair ground type thing every time i walk past seem to have lost my legs!.
I go every week, not had any dead things to pick up, but they do have a dog, the focus thing sounds important, perhaps as this one is not flat it over comes the spot focus prob?
Try your local glass merchant or B&Q etc.
Two other considerations:
(1) Be careful what's reflected in the mirror - seeing yourself reflected, e.g. at the edge of a seating area can be very distracting / off-putting to some people. Best arranged at an angle to reflect a view of another part of the garden, or used e.g. at the end of pergola, especially if covered with a "fretwork" tracery, or partly masked with planting, to give the impression of further depth;
(2) Be careful of the distance from which reflections are seen - any slight imperfections in the glass/acrylic, or any bending/straining from its mounting board are magnified with distance - so reflections may look fine from a couple of metres distance, but can look like the fairground's distorting "house of mirrors" when seen from say, 10 metres away.
I have a 'fibrecast' perspective arch mirror in my own garden which makes a great feature giving the illusion of a garden continuing beyond the brick wall it's mounted on. Very pleased with it. The mirror is slightly convex so no dead birds yet - and not expecting any anyway! Secured it with some smart brass screws with guide holes in the frame - worked a treat.
ps - here's a link though may be cheaper from manufacturer direct - http://www.fruugo.co.uk/perspective-outdoor-garden-mirror-large/p-1...