How many are you doing that cost is an issue?
The flock plan we received from our original vet (we changed when a closer one opened up), called for at least six samples a year, at £12 each, which soon adds up for a small number of sheep.
In the end I bought an old 1960s microscope on ebay,
and am in the process of learning to do my own FECs. It's really not that difficult, and once you have the kit (mine cost £75 all told), you can do as many tests as you like. I do intend to split samples and send to the vet occasionally as a cross-check too though.
We had some squitty lambs last week, and it took me fifteen minutes to run a couple of samples (both clear). To check I wasn't doing anything wrong with the microscope, I nipped over the fence into our neighbour's field and stole some poo..... they were absolutely riddled!