There are quite a few taxidermy catalogs available online. If you go to their website they often have photos of the form plus mounts on particular forms. As mentioned previously, a good taxidermist can alter to your particular tastes and needs.
You may ask your taxidermist if he'll allow you to take the form home and place it in the location prior to adding the skin to make sure everything is perfect.
In regard to sheep...they are super special critters and tags don't come easy. You generally get what you pay for with taxidermists. You may find it's worth every penny to pay a little extra $ for quality work, forms, eyes, tanning, etc.
My recommendation for sheep is either wall/table pedestal or life-sized! Every time you look at your sheep it's exciting to look at a fantastic mount and re-live the amazing memories!
