Ill throw out some options for you:
1st option:
Stick one of these in your server:
https://www.ebay.com/itm/234987470364
and connect it to this tower form factor “disk shelf” with both cables
https://www.pc-pitstop.com/8-bay-bare-12g-jbod
That would be a direct connect to each drive, no expander and the most you can connect is 8 drives with an HBA that is “-8e”. That is your cheapest option with high performance, though gives you less drive bays than you want and is not rackmount like you want. Costs $400 and gives you 8 drive bays. If you used 18TB drives that would give you a max of 144TB
2nd option:
a larger HBA:
https://www.ebay.com/itm/374588682817
and this chassis:
https://www.pc-pitstop.com/15-bay-4u-12gbs-sas-sata-trayless-rackmount-jbod-jr1512t
I don’t know if your server has a pcie expansion slot that can take a full size card though. You might only be able to use low profile cards so maybe this one wouldn’t fit. This gives you rackmount but is expensive. You have a direct connection to each drive still, no expander, but that isnt exactly necessarily a good thing. You probably cant use all that bandwidth with regular hard drives.
3rd option:
You can use the first -8e HBA I linked with this chassis and connect to all 15 drives at a combined total of 24gb/s over two cables. This one has an expander in it:
https://www.pc-pitstop.com/15-bay-12g-trayless-rackmount-sas-expander-jbod-w500w-psu-er1512t
Also expensive though.
4th option:
https://www.ebay.com/itm/234987470364
plus an expander card:
https://www.ebay.com/itm/134431659639
and this chassis:
https://www.ebay.com/itm/155456169623
This is your 2nd cheapest option, but is used hardware. That is fine in most circumstances and shouldn’t be a big deal.