I have a camelbak thermalbak 3L that I bought while back without a bladder, I found new bladders for 35-38$ I really don’t want to spend that much . Has anyone used a different bladder that works well ?

A Source hydration bladder looks like it might fit, but a new one on Amazon is going to run you around $30.  

Some of the off-brand Chinese hydration bladders I saw on Amazon might fight, but I don't know if they would leak or not.

Camelback bladders are gtg.
Lots of other bladders are just as good.  I have a Blackhawk hydrastorm bladder in my Camelback mule so far so good.  Cheap bladders don't last,  Camelback bladders used to not last so good (for the old ones) it sucks when you're miles from nowhere on a mountain bike and a bladder gives out.
I just simply wouldn't skimp on a quality  bladder.

You need to measure the diameter of the hole in the bladder carrier in order to select a bladder with matching fill-neck diameter.  Camelbak is your best bet for this, although there may be some other good bladders that will match.  Suggest only buying NEW bladders from trusted vendor.  You also want to make sure you get the "Hydrolink" drinking tube and mouthpiece.  Older units might lack one or both.

Note neck size: https://www.camelbak.com/en/military/gmi-packs/reservoirs/M04014--LongNeck_3L_Reservoir?color=4ddc27dc0b694fd39b8b6e563cc15434

This should work;  $26.99 shipped: https://www.newegg.com/p/0DR-0007-00031?Item=9SIA5D52JA8987&nm_mc=KNC-MSNSearch&cm_mmc=KNC-MSNSearch-_-SG%20-%20Storage%20%26%20Packs-_-Camelbak-_-9SIA5D52JA8987&source=region&msclkid=b27dea5b1c6a1e933d14b5392408159a&gclid=CMSSqdSH7usCFbkHiAkdSAUFnw&gclsrc=ds

Drinking tube insulators/covers and bladder insulators from https://www.hydrationtubecovers.com/

Disclaimer: no financial interest.