BPN-SAS2-846-EL1
BPN-SAS2-846-EL2 will also work, but may cost more since it has a second expander chip (for dual ported SAS drives only)
You can also go with a direct backplane and use a separate SAS expander or three 8port controllers, but the cabling gets messier and people are charging almost as much for the direct backplanes as the ones with the built in expanders these days:
BPN-SAS-846-TQ
BPN-SAS-846-A
Unfortunately there are tons of chassis' with the old SAS1 expander backplanes out there, so there are a lot of people in your position. There is not a lot of supply of the good backplanes, so $200-300 is the going rate these days here or on ebay for any of the above models, under $200 is definitely a great deal. I wouldn't pay more than $300 for one, your threshold may vary.
AVOID these:
BPN-SAS846-EL1
BPN-SAS846-EL2
BPN-SAS846-7EL1
BPN-SAS846-7EL2
Sometimes the model numbers are represented with more or fewer dashes - the important part is that if you want an expander backplane, it should be BPN-SAS2[something]EL[1 or 2] or if you are lucky enough to find one, BPN-SAS3-[something]EL[1 or 2]
Good luck!