Forget LinkedIN unless you have a large following of people doing roles similar to those you are recruiting, or you are willing to wait a while for the right person to come along, it is slow burn not instant result.
I did 27 years in IT & Tech recruitment with a bit of Accounts as well so tell me the type of role i will tell you where to advertise (and likely cost)
You can always take your chances with Indeed, it is free but you get what you pay for which is basically a mass of applications many of which will be irrelevant (people who are signing on and needing to prove they have made x applications per week). Reed have a similar option (was knows as Reed Freecruit) you will get a canvassing call from one of their branches but that is a small price to pay.
If you are writing an ad remember AIDA (courtesy of an event i went to years ago hosted by the institute of practitioners in advertising)
A ttention (on an online ad this is using the words people are searching with)
I nterest (the wording describing the job which makes the applicant think it applies to them)
D esire (the reasons why they should apply to you, not just £ but location, working environment, training or advancement opportunities)
A ction (how they apply, by when, what format etc)