Got a product and maker you want to use?
Depending on where you live , if the company is currently open given the situation and local availability you can purchase some at the local auto paint stores or online. Done it both ways.
For recommendations use the search there are dozens of threads showing examples and posts of what worked for other members. Some applicators can be hundreds of dollars or you may be able to get the paint shop to lend you one.
Did, recently, look up the Sludge product I and others have used and it appears they have changed the product name and it doesn't take a special gun in fact I didn't use a gun or special applicator for the rear wheel wells and trunk area though I did a final spray top coat with another rattle can product successfully. Pictures of that application are in an older 69 GT350 restoration thread and in other threads as mentioned