Where can I find free large sunflower designs to paint on a blue bed
sheet. I need the design to be alligned in a line for the bedsheet and
any thing matching for the bed sheet.

I do not have any formal training/experience on fabric painting so
where can I get tips for the same?

To me this seems like a multi step process. 
But I think I can help with part of it.

First you would have to find a picture that you like.
Next you need to get the picture (or it's impression) on to the sheet.
Here you have a few choices. 

If you are starting with a print you can use an overhead projector,
hang the sheet on the wall and trace the design on to the sheet.

If your original is digital you can use a paint program to blowup the
image and do a 'Poster print' (that is making the picture take up 4,
6, 9 or 16 pages)

You could use Iron on transfer paper and just Iron the picture in
place, but I would not like sleeping under such a sheet.

If you print on plain paper the you could tape the pages together into
one large sunflower picture. Now I would tape that to a picture window
or sliding glass door. Then hang the sheet between you and the big
picture.
Now trace the image (or it's outline) to the sheet.

Not good enough?
Ok I have one more way to transfer an image. You can print as before
but do a mirror image and use Glossy Premium quality Photo paper for
Laser printer, but be sure to print with in ink jet printer. (I have
seen this done on HGTV's Carol Duvall Show but I have never tried it.)
Do this work on a flat surface. Spray the cloth sheet with water, lay
the paper image side against the bed sheet, spray the back of the
paper with water and rub the back of the paper with a craft stick.
This Should transfer the image.

Now it's time for selecting your fabric paints and brushes.

At this point I am out of my depth. Perhaps someone else can help find
you a good source for the How to paint on cloth. But at least you have
an outline prepared.

I hope I was of some help.