I want to shop in the currency that will be cheapest in CAD at any given time.

How do you plan to do this? If you are using a debit or credit card on a CAD account, then you will pay that bank's exchange rate to pay for goods and services that are billed in foreign currency.

If you plan on buying goods and services from merchants that offer to bill you in CAD for items that are priced in foreign currency (E.g. buying from Amazon.co.uk GBP priced goods, but having Amazon bill your card with equivalent CAD) then you will be paying that merchant's exchange rate.

It is very unlikely that either of these scenarios would result in you paying mid-market rates (what you see on xe.com), which is the average between the current ask and bid prices for any currency pair. Instead, the business handling your transaction will set their own exchange rate, which will usually be less favorable than the mid-market rate and may have additional fees/commission bolted on as a separate charge.

For example, if I buy 100 USD worth of goods from a US vendor, but use a CAD credit card to pay, the mid-market rate on xe.com right now indicates an equivalent value of 126.97 CAD. However the credit card company is more likely to charge closer to 130.00 CAD and add a foreign transaction fee of maybe $2-3, or a percentage of the transaction value.

Alternatively, if using something like Amazon, they may offer to bill the CAD credit card in CAD for those 100 USD goods. No separate foreign transaction fee in this case, but they are still likely to exchange at the less favorable 130.00 rate instead of the mid-market rates.

The only way you can choose to pay in the cheapest equivalent currency is if you already have holdings of all the different currencies. Then just pay using whichever currency gets you the most bang for your buck. Unless you are receiving payments/wages in multiple currencies though, you're still going to have to refill these accounts periodically, thus incurring some foreign transaction fees and being subject to the banker's exchange rates.

I want to shop in the currency that will be cheapest in CAD at any given time.

How do you plan to do this? If you are using a debit or credit card on a CAD account, then you will pay that bank's exchange rate to pay for goods and services that are billed in foreign currency.

If you plan on buying goods and services from merchants that offer to bill you in CAD for items that are priced in foreign currency (E.g. buying from Amazon.co.uk GBP priced goods, but having Amazon bill your card with equivalent CAD) then you will be paying that merchant's exchange rate.

It is very unlikely that either of these scenarios would result in you paying mid-market rates (what you see on xe.com), which is the average between the current ask and bid prices for any currency pair. Instead, the business handling your transaction will set their own exchange rate, which will usually be less favorable than the mid-market rate and may have additional fees/commission bolted on as a separate charge.

For example, if I buy 100 USD worth of goods from a US vendor, but use a CAD credit card to pay, the mid-market rate on xe.com right now indicates an equivalent value of 126.97 CAD. However the credit card company is more likely to charge closer to 130.00 CAD and add a foreign transaction fee of maybe $2-3, or a percentage of the transaction value.

Alternatively, if using something like Amazon, they may offer to bill the CAD credit card in CAD for those 100 USD goods. No separate foreign transaction fee in this case, but they are still likely to exchange at the less favorable 130.00 rate instead of the mid-market rates.

The only way you can choose to pay in the cheapest equivalent currency is if you already have holdings of all the different currencies. Then just pay using whichever currency gets you the most bang for your buck. Unless you are receiving payments/wages in multiple currencies though, you're still going to have to refill these accounts periodically, thus incurring some foreign transaction fees and being subject to the banker's exchange rates.

Where can I lookup accurate current exchange rates for consumers?

It depends on who will be handling your transaction.

Amazon will tell you at the checkout what exchange rate they will apply if you are having them convert a bill into your local currency for you.

For credit/debit card transactions processed in a different currency than the attached account, you need to look at your specific agreement or contact the bank to see which rate they use for daily transactions (and where you can obtain these rates), whether they convert on the day of the transaction vs. the day it posts to your account, and how much they add on ($ and/or %) in fees and commission.