1) The free loan agreement
A simple search for "free loan document" will reveal a number of free loan agreements. Should you use them? Perhaps. If you're comfortable starting with a free loan agreement and making the necessary changes yourself and understand the impact of those changes then certainly this is a low cost approach, assuming the loan agreement fits your needs.
2) The low cost loan agreement
There are a number of services out there, both online and offline, that will provide you a template loan agreement form, very similar to what you might find with your free loan document search. The main difference between these providers and the free document providers is that there is assumed to be work that goes into maintaining the document and that it is suited for use as a basis for an agreement. The old adage "You get what you pay for", might apply to this situation as you can at least ask questions of the low cost provider and expect that they have an interest in ensuring you receive the right product for your needs.
3) The low cost loan agreement with added services
The next level of offering is a low cost loan document with some additional services to help with the creation of the document as well as helping with the repayment part of the lending process. For a slightly higher fee than just a blank loan document you can receive a more customized loan document as well as additional services like email payment reminders and/or payment tracking.
4) The lawyer
The most expensive option of any of them is to simply use a lawyer. Sometimes this is the best option if you need to create a formal document that is non-standard or if you just want to ensure that the document you are creating is appropriate for your situation and that you're not forgetting any important clauses. A less expensive compromise might be to obtain a free or low cost loan document and have a lawyer review the contents before finalizing and signing your agreement.
Regardless of the method you choose to create your formal loan document it's certainly good practice to document all terms of any loan you agree to in order to protect the interest of all parties and prevent any misunderstandings down the road.