Just buy a light scarf and put it in your purse. You can use it to cover whichever body part you have hanging out. That’s what my wife does.

Walmart is not present in Italy, so you’d be looking for a long time.

Good idea… thank you.

Remember modesty for your whole body, male or female. The dress code rules when visiting churches are simple: cover your shoulders and knees, and you must wear shoes.

Here's one of many links with more details -- https://www.romecabs.com/blog/docs/how-to-dress-when-visiting-romeitaly-churches/

Yeah, a wide scarf is probably the best defense against violating the norms. But it comes down to not being a hard and fast rule, but a sign of respect. But really, there is no detailed list of regulations, no one with a ruler, checking your shorts or skirt, no one checking as to where your shoulder joint sits against the cloth. Most smaller churches will not have anyone there to scold. You are entering a house of God, use your judgement, like you would expect people entering your place of worship.

Thank you all. I appreciate all your words of wisdom.

Most of religious sites in Italy state no bare shoulders. I see this
as no tank tops, strapless, or straps on blouses, but does this also
mean no sleeveless tops?

I'll make this easy: it means no sleeveless tops for men OR women. Short-sleeved tops are fine as long as your midriff is covered and you aren't showing an immodest amount of décolletage. Same with your knees; men and women, they need to be covered.

Packing along a scarf to cover the upper arms if wearing a sleeveless top is a common remedy.

It appears that sleeveless top are not OK for women, though this information is not directly from a Vatican website:

Dress Code for Women
Vatican City adheres to the same modesty standards for both men and women. However, there are some occasions when women must adhere to a specific dress code. According to the St. Peter's Basilica Dress Code, women are not permitted to wear sleeveless tops, crop tops, or tight shirts. Shoulders should be covered, and if wearing a skirt or dress, it must cover the knees.

#

Dress Code for Men
Consider leaving the ripped jeans and sweatpants at home. You can't even wear a baseball cap or any other type of hat inside, so don't carry one. Trousers and shirts are good examples of complete clothing that cover the majority of your body.

Getting around Rome during the summer months can be difficult. During summer, men are allowed to wear shorts, as long as the knee-length is observed. Linen trousers and t-shirts are also acceptable.

#

Dress Code for Mass
During mass at St. Peter’s Basilica, men and women are required to cover their knees and shoulders. Men can wear trousers and shirts, while women can wear long skirts or dresses, both of which must cover the knees.

Women are allowed to wear hats for mass, however, men must take their hats off before entering the Church.

#

What to Wear to St. Peter’s Basilica
Shirts or blouses that cover your shoulders: Plain t-shirts, formal shirts, or full-sleeved blouses or tops are permitted.

Trousers or skirts that cover your knees: Pants that cover your knees and long skirts or dresses can be worn inside the Basilica.

Flat shoes: Go for flat shoes or sneakers. Don’t wear heels, open sandals, or flip-flops, as it is a sacred area.

Loose clothing: Visitors are encouraged to wear clothes that are casual and loose-fitting.

Weather-appropriate clothes: In summer, loose linens will work, however, sweaters & jackets are a good choice for winters.

#

Here's the Non-Vatican source link from which I copied the above information. That's not the only website I found that explicitly says sleeveless tops are forbidden.

Flat shoes: Go for flat shoes or sneakers. Don’t wear heels, open
sandals, or flip-flops, as it is a sacred area.

I'll disagree with this one. I've never heard of any of the churches having an issue with heels or sandals. My goodness, there are Catholic monks whose standard footwear expose their toes! :O) One other recommendation I've seen is not to wear t-shirts or other apparel with violent or lewd graphics or text. That makes sense.