AFAIK, if you have a legitimate prescription, the pharmacy has to fill it. Well, unless it's one of those controversial religious/'ethical' things like the morning after pill, but this doesn't fall anywhere in that spectrum. I'd suggest speaking to the chief pharmacist. If s/he won't help, I'd try the store manager or, if it's a chain store, escalating up the chain of command. I've had pet prescriptions filled at probably 40+ pharmacies, and not one has ever blinked at filling anything I brought to them. That includes syringes, needles, narcotics, steroids and barbituates; it also includes several medications that are very heavily regulated by the FDA and are of extreme interest to the DEA.
Do you happen to have a WalMart anywhere near you? I've used them in four different states and never had a problem getting syringes from them. Sometimes I have to let them record my driver's license number or sign a registry or answer some questions from the pharmacist (what type of insulin, how much and how often), but they've never hassled me about them. They also have the 3/10cc syringes, and they come in a lovely 31 gauge size -- the thinnest insulin syringe they make, so it's less painful. IF you have a WalMart, you might want to call to see if they have them in stock, as not all stores carry them. However, when they haven't had them in stock, I just ask them to special-order them for me and they're almost always in the following afternoon.
The syringes you have on hand use the same-size unit as the ones the vet wanted you to get. so 1.5 units is the exact same amount on either syringe.
The differences between the syringes are:
* the diameter of the needle (29 vs 30 gauge);
* the total amount of insulin the syringe can hold (1/2cc vs 1/3cc);
* the amount of space a unit takes up in the syringe (it's often easier to accurately measure out smaller doses in smaller syringes);
* and some brands of 1/3cc syringes have half-unit markings, whick make it easier to measure half-unit doses.
Just do the best you can in measuring the dose until you can get the syringe situation sorted out, and remember that if you have any doubts as to the accuract of the dose, it's better to give too little than too much.