There are many sources for mutual funds. You can buy them through most stock brokerages, including on-line brokers. You can buy them through banks and other financial institutions. Some banks have their own funds. There are a number of companies whose main business is the selling of mutual funds and other similar financial instruments.
Some of the better known are T. Row Price, Dreyfus, Vanguard, and Fidelity. You can buy funds directly from these companies. One of the advantages of buying a fund from the company that manages it is that it is very easy to switch to a different fund within that company. You can do it online.
There is a difference in fees and that should be taken into account when buying funds. Even if you buy a fund from a mutual fund company, there are various fees associated with that fund and it varies from fund to fund to a large degree. Heavily managed funds tend to cost more than index funds and others that are more passively managed or not managed at all.
If you have a job with a 401k or 403b account, it is likely that your retirement account is with one of the fund companies. At my job I had the choice between Fidelity, Vanguard, and TIAA/CREF. At different times, I had money in funds in all three companies. When I retired, I put all of my money in one fund with Fidelity and opened an IRA. At present, my IRA is all in just one Fidelity fund.
Wherever you buy your mutual fund, you can buy a diverse or specialized fund or you can diversify your money yourself by buying into more than one fund. The choice of where to buy your mutual fund should take into account the cost of the fund and convenience as well.
I am partial to Fidelity because they were so helpful in setting up my IRA as well as getting my funds from my work account to my Fidelity account. But you will find most fund companies and other companies that sell funds quite eager to help you find the fund that will work best for you and get your fund account set up with a minimum of hassle. After all, they want your money.