If you want to fund your accounts with some token (for example DAI) after forking mainnet, there are two things you can try.
One is to impersonate an account that has that token, and either use that address or send tokens from it to one of your test accounts. For example:
const [signer] = await ethers.getSigners()
// impersonate account; replace with an address that actually has your token
const addressWithTokens = "0x49a77c5703e877759307175756E6EFDb3669ed77"
await network.provider.send("hardhat_impersonateAccount", [addressWithTokens])
const impersonatedSigner = await ethers.getSigner(addressWithTokens)
// create the token instance
const token = await ethers.getContractAt("ERC20", tokenAddress)
// connect it to the impersonated signer and send it to your signer
await token.connect(impersonatedSigner).transfer(signer.address, "1000000000000000000")This assumes that the impersonated account has enough ETH to make the transfer. You can send ETH to it first if you want, but keep in mind that if the account is a contract, then it might not accept receiving ETH.
Alternatively, since the test accounts start with 1000 ETH each, you can use any on-chain exchange to swap that ether for the token you want. How to do that depends on what exchange you want to use.