I'd recommend not doing anything related to safety on the cheap. Racing seats are expensive because they are engineered to keep you safe in the worst instances.
There are two things to check when buying a racing seat;
1. Safety - The best racing seats are FIA homologated. This means that the seat is safe for track use and won't come apart in an accident. Without this, you are trusting the manufacturer (often some unknown Chinese company) with your life. This was not for me.
2. Mount - Even a seat that comes with sliders will usually require a custom mount as an interface between the slider and the mounting holes in the chassis. You'd think that there would be some kind of standard but there isn't. You might get lucky and find an existing mount that works but that is more the exception than the rule.
Also, the mount can't be made from whatever spare metal stock that you might have laying around the workshop because the mount and the bolts used need to be 110% capable of handling the loads that occur in an accident. Hence, you might want to have a shop familar with racing do this work for you.
Pics: Sparco Pro2000 Plus seats and 6-pt Schroth harnesses. The stock seat belts and receptacles are retained so I don't have to use the 6-pt harness on the street. Ingress/egress is a bit more difficult in a seat like this (note where the black fabric is worn off of the drivers left side bolster from rubbing each time I get in/out) but this setup is PERFECT for track driving.
The lower pic shows the mounting system: Racing seat to Brey-Krause side mounts which attach to the slider which attaches to a custom floor mount.
Safety isn't cheap. Seats + side mounts + sliders + custom floor mount + labor = $2,400. Racing harnesses + mounting = $800.
