i find that becton dickinson (bd) produce the best syringes around. they do numerous product lines for diabetic syringes and insulin pen needles.
as of june 2000 diabetic syringes are available free of charge for residents of nsw, act, qld and vic through what is called the national diabetic supply scheme (NDSS), so you may want to think about that before requesting syringes for your "diabetic" mother.
various drug and alcohol centres around sydney and other capital cities have vending machines which stock what are commonly called "fit kits," containing a spoon, alcohol swabs, sterile water and needles. these kits are around $2 and also double as a disposal container for your used sharps. you can also find needle exchange centres around the place which will gladly exchange old for new, no questions asked, and also make fresh needles available to you.
i would recommend a b-d ultra-fine needle, 29 gauge (0.33mm X 12.7mm), 1mL. they are available in boxes of 100, not sure what the price is nowadays, but it used to be around $5 a while ago. you may want to experiment with longer or shorter needles (they range from 8mm to 12.7mm but i find the longer ones do just fine). i don't think you would prefer a higher gauge needle unless you're a sadist, but hey, go ahead if you want! it may help if you just walk in with that written down on a piece of paper, say your mum isn't on the NDSS, and act dumb. it's not illegal to purchase syringes or use them to self-administer drugs...
have fun!