I picked up from Harbor Freight with one of their 20% coupons a 4,000-lb non-swivel snatch block for something like $5.
The spindle or axle bolt is metric and thread-locked in case you want to get it apart to thread a wireline thimble and U-bolt clamps through to wrap around the sheave.
If you're doing serious work and / or have other limitations (legal implications of contracting or working for others that might be inclined to litigate), stay with the name brands like McKissick or perhaps Peerless.
The name brands sure aren't inexpensive, but they're built hell for stout and I've never had one fail. I'd be real skittish of some of the imports on the rack at Tractor Supply or Atwood's or Orschlien's.
All my McKissick swivel-hook blocks have a heavy wing-nut style release that allows the side plate to rotate 'open', making rigging up (and down) much simpler and faster.
Be cautious using a snatch block for if something fails or lets go, anyone in the line of fire can get terribly hurt so quickly you'll never know what happened.
Along those lines and if your block doesn't have a swivel hook take a light bind on the load and look closely how the block aligns with the load and if your loadlines are getting crossed or tangled before pulling a full strain.
Along those lines make sure the wireline you're using is of good quality and in good condition, tested if possible. Make certain your hook safety latches are present and functional. Know your load.
Please post back your continuing experiences so we may all learn.