You can buy Sylgard just about anywhere. At the moment, prices for Sylgard are high, as there seems to be a worldwide shortage of Silicone elstomers.
The best price I was able to find was at Ellsworth for $109. Here is a link:
Remember, a 1/2Kg of Sylgard will yield about 7 tanks, max for a Form 1 standard tanks, assuming you do them all at once. The average Form 1/1+ tank will take ~65-67g. The ZVat takes about 56-58g.
If coating Form 2 tanks, those are about 105-108g, so you’ll only get about 4 tanks out of the Sylgard container.
There are other alternatives to Sylgard as well which are cheaper, such as ML Solar, or Qsil 216. In fact Qsil is about $40 for 1 LB of the stuff.
Last but not least. Don’t buy now thinking that you’ll recoat sometime in the future. This stuff has a shelf life of about 2 years from the date of manufacture, after that it will not cure properly. So make sure you check the date of manufacture and use it before it expires.