Anyone experienced in getting eye contacts online? Had issues searching and using correct prescription?

So, need advice on where and how. I have found websites that do contacts online based on your prescription, but cannot find accurate ones for my prescription, I also only have a prescription for glasses (I can't currently get a contact prescription) so I know it might be a little different anyway - it's okay if they aren't perfect, I will only where them occasionally for a couple hours at a time, but how do I convert them as accurately as possible under the circumstances?

My prescription for glasses is left eye -5.75 and right eye -6.25 cylinder -.50 and axis 0.75 (as in I only have astigmatism remaining in my right eye not left anymore).

Online I noticed the increments go up/down .25 UNTIL you REACH -6.0 then it goes -6.50, -7, -7.50 etc

and the axis goes by 10s, 50, 60, 70, 80 (mine is 75)

Since glasses sit a few mm away from eyes and need to be stronger than contacts should I go .25 less anyway? So left eye -5.50 and right eye -6.0? Should I pick cylinder -.75 (since that is the lowest available) and axis 70? (or 80?)

Again, I know it might be perfect but any advice on getting it as close as possible would be helpful - I will get the contact prescription eventually, but in the meanwhile need to work with what I've got.