C.T.
I understand your frustration about the expense of Childcare and not being able to find a reliable replacement since your circumstances are allowing you the opportunity to not have to have your kids in Childcare regularly is great.
However, finding a sitter to be at your whim is a big request! There's not much reliability on the babysitter's end and if they find another job with more stability, can you blame them for pursuing?
My advice would be to find out if your current Childcare will allow your kids to drop in on days you need care. If not, find one who will. Usually the cost is a little more but not always. Or work out a deal for their care 6 weeks on & 6 weeks off. You'll still be paying only half of what you used to pay.
If you want to continue to pursue a babysitting option, consider a service who takes calls and can fill your order as needed versus using a free website. I don't know how many of these places exist, but I would imagine Brightstar could help you. That's where I would start.
Good luck!