Cash is king on Lombok. Wherever you go you will need it, whether to pay for parking, entrance to museums, tips, or taxi rides, or to buy knickknacks, you will need to have low denominations of cash. Although Rp100,000 bills are useful for high-priced items, smaller shops and taxis do not carry large amounts of change. There seems to be an expectation that the customer should provide appropriate change, rather than the other way round.

ATMs are everywhere in Lombok.
Everywhere. Even on the Northern Gili Islands are several ATM's.
On the mainland you'll find ATM's in every touristic place. Even
in some supermarkets.
Withdrawals can be made with credit cards and some debit cards on the Maestro and Cirrus networks. Most ATMs dispense money in multiples of Rp50,000 which is extremely annoying when withdrawing large amounts of cash. A few machines will dispense Rp100,000 notes. All ATMs are clearly marked which denominations they pay out.
Some banks will only allow maximum withdrawals of Rp1,250,000 at one time but will allow up to three withdrawals in a day. Others will allow Rp3,000,000 at one time with a maximum of Rp6,000,000 withdrawal in a day. The best ATM we have found is Permata
Bank, where you can withdraw Rp3,000,000 in Rp100,000 notes up
to a maximum of two withdrawals.

All high-end hotels, shops, restaurants, and nightclubs accept
credit cards. Visa and MasterCard are the most commonly accepted cards but some do take American Express and Diners Club. Merchants may charge a 3% to 5% surcharge for credit transactions. Payments are usually in rupiah but some companies have been known to charge in U.S. dollars, particularly large hotel chains. If this is the case, ask them to clarify what exchange rate they are using as you may be better off paying in cash.
Tips for changing money with money changers (there are also many unscrupulous vendors):
Make sure you do your own calculations. Do not rely on the staff. Some calculators can be tampered with.
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Check to see if there is a commission fee. Be cautious that you may have a good rate of exchange but have to pay a hefty commission rate. If the commission is low, ensure that you are getting a good exchange rate.
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Count the money yourself. Better still, count it twice. Do not pass it back to the staff to recount as you may find a few notes missing after leaving the shop.
The money changers should give you a receipt. If they don't, insist.
Be warned of counterfeit bills. If a note doesn't feel right, ask for another one. Do not accept any damaged currency.
Be sure to count zeros on a note. Rp10,000 is roughly equivalent to US$1 and Rp100,000 is US$10.