For most first year businesses the list is short and predictable. Accountants and bookkeepers, attorneys handling formation, insurance brokers, commercial property agents, business bankers, and the trades adjacent to yours. Each of them meets your future customers before you do, at the moment those customers are making decisions.
The reason this works is that referrals travel along established trust. Somebody who has just paid an accountant to set up their business will accept that accountant's recommendation about a website almost without evaluation. You cannot manufacture that trust and you can borrow it.
Start with the professionals you already use, which is the step people skip. Your own accountant, your own insurance broker, your own attorney. They know you, they have seen how you work, and the conversation is considerably easier than a cold one. Most businesses have three or four of these relationships and never think of them as a channel.
Approach with what you can give rather than what you want. The message that works says you get asked for recommendations in their field and would like somebody to point at, since presumably they get asked in yours. That is an offer of reciprocity rather than a request, and it is easy to accept.
Be specific about who to send you, because vagueness is why most partnerships produce nothing. A partner who understands you work with businesses in their first year will remember you at the right moment. One who understands you do digital work will not, because that describes too many people.
Give them the sentence you want repeated. A referral is carried by somebody who does not do your job, in a conversation you are not present for, and it survives only if it is short and concrete. If they cannot describe what you do in one line, they will not attempt it.
Make the first exchange real rather than notional. Send them something useful, refer somebody to them, or answer a question they have. A partnership that begins with a genuine act works considerably better than one that begins with an agreement to help each other eventually.
Follow up promptly when a referral arrives, because that person staked something on you. A slow response reflects on them rather than only on you, and it is the fastest way to stop receiving them. Tell them what happened afterward, whether or not it converted.
Then keep the number small. Three or four genuine partnerships that produce work consistently are worth more than twenty acquaintances who have your card, and the difference is entirely in how much attention each relationship receives.
Ask your existing customers who else they use, which is the fastest way to build the list and costs one question. The names that come up repeatedly are the businesses already positioned where you want to be.
Be patient about the first exchange producing nothing. Partnerships form over months because they depend on the right customer appearing in front of the right person, and a partnership judged after six weeks has not been given the chance to work.
Meet in person where geography allows, because these relationships form considerably faster face to face than over email.
Keep a short list of the people you would refer to across the adjacent categories, before anybody asks. Being able to name somebody immediately is what makes you the person others come back to.
Thank people specifically when a referral converts, and tell them what the work turned out to be.