you can get it at almost every reptile or pet store. a little bit lasts a long time. still going off some bulk i bought 5-6 years ago. not even half-way thru. as long as it stays dry it doesn't expire, like salt.
also you need to get a reptile multi-vitamin. i am currently trying some jurassi-vite. however i honestly did not use this until more recently. prior to that i gut loaded and bought this stuff in the form of the gut load for prey. however that was a lot more work and i don't keep near as much now.
although the crested geckos can and will live entirely off the CGD. that is almost only found online and would be best bought in bulk. if worried about fraud use a prepaid credit card. also the bank can take care of fraud if it happens, but still be careful. if not 18 then oh well. have someone else get the product for you.