Simplyeyeglasses.com can do a correction up to a -20 on their online
form. My wife has a strong prescription as well and she had a pair of
glasses relensed there and they did a good job. They sell designer
frames and are more expensive than many of the other online shops, but
at least they will do it.
Another thing you might want to do is ask the customer service
departments of any online glasses shop you are looking into if they
can handle the prescription. I contacted one in regards to my wife's
prescription and they told me to just scan it and email it to them and
they'd let me know. Since she just wanted relensing and not new
glasses, she went with Simplyeyeglasses.com so we didn't follow up
with the other place.
Hope this helps.
RedStickHam