Milton,
Here's my 2 cents (opinion, take it as you will- feedback is always welcome)-Yes you can buy prequalified welding procedure specifications, I've not heard of AWS selling the procedure qualification records. You can use these purchased WPS for welder qualification test and for you specified fabrication WPS's, although these that I've seen are a bit conservative for actual fabrication, my thoughts are the best thing to do is check into what your most productive {quality and quantity}Weldors are using. Identify the required parameter variables [there are WPS checklists available] once you know what are the essential and non-essential variables record what the production welders are using then check against the section 3 prequal requirements if the material both base and filler, joint design, welding parameters, and the remaining items from the checklist are within the specified ranges and your welders are qualified document it,  then just for piece of mind test them against the usage needs, and do some guided bends, and macro-etch a few specimens then  have your engineers approve them and verify with your customers that these are acceptable to them and you'll have WPS's in place and can use these to further develope and expand. The logic I follow for this is that in developing your own you'll have a better time getting buy-in from your workers and the values will be practical for the applications.
When thing don't fall into the prequalified ranges this can be a different matter, depending on what you have available for a testing lab sending the test specimens out may be the cheapest way, but once again if you have many to make it could be a practical investment in acquiring the testing lab equipment and getting the appropriate certifications and qualifications to do them in-house if this option is available to you I feel that the learning that you and your welders will get will be the biggest pay-off.
Sometime we weldor types have huge egos and since seeing is believing things can ease the pain of finding out or way isn't the best.