I completely understand your situation.
I wouldn't recommend treat unless it is necessary(confirmation of parasites). Do you have a local college with a vet. course or other science related courses that might look at a fecal sample for you. You might also find a reptile vet. and call him explaining what you need(fecal test only). Some vets. are sympathetic to these situations and will help out.
I know that Panacur is relatively safe but I still don't recommend any meds. unless they are needed.
I know you said you've tried everything but I can't imagine any garter turning down cut-up worm with crushed fish mixed in. Try placing a dish of this under the hide and see what happens.
If you find a vet. that will do it at a minimal cost let us know, maybe some of us can help you out