Right up our alley :-)
We have been researching this for over a year now and it has not been easy! Well, the CA was rather easy but the ALK part was a nightmare.
Bulk CA:
http://www.reefsupplies.ca/online-store/bulk-reef/
There are many many many chemical suppliers of "quality" bulk soda ash. Do a google search and you'll find tons. The problem is that their quality is not what we in the hobby consider quality. They did not intend to grow sensitive sps corals :-) In the past year we have brought in over 100 pounds of soda ash samples with 97%+ pure COA sheets. We field tested every single one and most failed. They failed in our test tank and our lab tests. Yupp, we sent some to a lab for testing.
The ones that failed did not show any real negative affects on SPS. SPS still grew but some coloration was effected. The real bad stuff melted some LPS corals yet SPS were not affected. Some of the cleaner stuff had positive affects on corals but caused cyano outbreaks.
A year down the line we have finally hit the nail on the head. We found super super clean soda ash that we will soon be selling in bulk. This batch passed every lab test with flying colours. It also had surprisingly low NPOC! The stuff is amazing.
Now we look forward to testing MgCl2.6h2O and MgSO4. Good times!
Ray