In a version 2.7.0, there was added a way to LV up your parnter's friendship LV by feeding them with items you can buy with your GOLD. So let's see where you can by those items, how many friendship EXP they give, and how much they cost.

Candy Bown

In 2.7.9 a path previously blocked by cones was unlocked and revealed rock puzzles and also a candy bowl from Undertale. However, someone before you knocked out the bowl and you can only grab two floor Monster Candy which only heal 5 HP instead of 10 like in Undertale and gives 2 Friendship EXP.

Spider Bake Sale

In Spider Bake Sale(which is located at same location in original Undertale, in the Ruins) you can buy 2 types of food: Creepy Crepe and Crawly Cuppa. Crepe heals 5 HP, costs 15G(or 8 if you do spider side-quest) and gives 2 Friendship EXP. Cuppa heals 10 HP, costs 30G(or 15 if you do spider side-quest) and gives 3 Friendship EXP.

However, when it’s October, Crawly Cuppa gets replaced by Candied Corn, which costs same amount of G, but heals 12 HP instead of 10, and gives 5 Friendship EXP instead of 3.

Grillby's

In Grillby's(which is located in the same location as in original Undertale, in Snowdin) you can actully buy food from Grillby unlike in Undertale. Here's the list of items:

Soda - Heals 8 HP, Costs 25G and gives 4 Friendship EXP
Fries - Heals 15 HP, Costs 50G and gives 5 Friendship EXP

Burger - Heals 20 HP, Costs 80G and gives 6 Friendship EXP

Salad - Heals 25 HP, Costs 100G and gives 7 Friendship EXP

Overall, the items are good for feeding your partners since they dont cost very much.

Tem Shop
Tem Shop is located in Tem Village, and Tem Village located down to the secret lilypad path in Waterfall - Quiet Hub. There you can buy Tem Flakes for 3G, Tem Flakes for 1G, Tem Flakes for 20G(which are all heals 2 HP and gives 1 Friendship EXP) you can also buy a Tem Key for a Tem Armor chest(which requires a LV75 or 8500 Spares(and you cant feed it to your partner)and costs 999G). Overall the most cheapest Tem Flakes costs 1G and gives you 1 Friendship EXP, so if you got a lot of time but almost no GOLD, you can feed all your partners with Tem Flakes. But there's also... items that costs A LOT of G and doesnt really give you much Friendship EXP that we will talk about it later, but for now.

Hometown

In 2.7.10, Closet Key previously unlocking path to Dark World in New Home became Silver Key unlocking Sans' basement, and in said basement, you can interact with machine to went to Hometown. In Hometown, there exist QC's diner where you can buy Hot Chocolate for 120 G, which heals 30 HP and gives 10 Friendship Points.

Bake Sale
Bake Sale located in the Card Kingdom almost at the end of the map and the items are being selled by Rudinn and Hathy, Choco Diamond and Heart Donut(the best healing items btw). Both Heals 35 HP(or 45 if you got LV20 for Choco Diamond and 750 Spares for Heart Donut), costs 150G and both give 8 Friendship EXP. I wouldn't personally recomend feeding partners with those items because you need to go through all the dialogue and it costs more then just buying 2 sodas. But there's also other items that are costs like a liver and gives not very much Friendship EXP compared to other items(which I was already talked about later), and those items are from...

Vending Machines
The vending machines located in different places each one for a main location(except New Home) and you can buy there drinks that either makes all your acts fill spare bar for 100% if you befriended that enemy, or increases your chances of finding gold or even diamond enemies, or even corruption battles for 30 minutes. So they're not really of a items for feeding items they're more a buffs for grind so that's why they cost that much GOLD. So again, I wouldn't recommend using those for feeding partners but if you want I guess for some weird reason(or if you REALLY have a LOT of GOLD), here's a list of them.

Pacikiss - Makes all your acts fill spare bar for 100% if you befriended that enemy for 30 minutes, costs 3500G and gives Friendship EXP 10

GLD BRU - Increased gold chances for 30 minutes, costs 5000G and gives Friendship EXP 12

Crystal Bepis - Increased diamond chances for 30 minutes, costs 7500G and gives Friendship EXP 14

Gamer Blood - Increased corruption rate, costs 9000G and gives Friendship EXP 15

And that's it for the guide, I hope it helped you choosing what items you should use for feeding your partners and just helped knowing where you can buy them, how much costs and what they can do. Thanks for reading.