G'day Sue,
Gow Gates is as good as anyone I guess...you can get them via e-mail on info@gowgates.com.au or their website is www.gowgates.com.au.
Unfortunately no meaningful Equine Insurance comes cheap and I am certain it would be prohibitively expensive to do what you're thinking of doing...as in covering them from day of foaling to fall of hammer which would be some 16-17 months or so on average. For example, leading up to a Yearling Sale, 100k's worth of cover for about 6 weeks will cost you around about $1,200.