The buy limit for each available funding method is listed under Account Limits. It's also visible on the buy page when you select that funding source.
Coinbase may have separate limits for the different account funding sources listed below.
Bank transfers (ACH, SEPA, GBP, Bank wire)
Instant purchases via bank transfers, enabled by account history and a backup credit card (available to select users in the US only)
Cash balance stored value (USD, CAD, EUR, GBP, etc.)
Instant buys (credit card/debit card, where available)
You can add funding sources on the payment methods page. You can also fund your Coinbase account with a wire transfer (not available in all regions) for up to the buy limit listed on your verifications page, without linking a bank account to your Coinbase account.