Spiders don't need to eat that much. In terms of food requirements, giant spiders would be fairly supportable by most environments.
As per Kleiber's Law:
For the vast majority of animals, an animal's metabolic rate scales to the ¾ power of the animal's mass. Symbolically: if q0 is the animal's metabolic rate, and M the animal's mass, then Kleiber's law states that q0 ~ M^¾. Thus a cat, having a mass 100 times that of a mouse, will have a metabolism roughly 32 times greater than that of a mouse.
The reasoning behind this is that smaller an animal, larger the fraction of their body mass consisting of structure rather than reserve. Structural mass involves maintenance costs, while reserve mass does not; ergo small animals respire faster and need more calories per mass than larger ones.
Assuming Kleiber's Law to hold true for these giant spiders, we can calculate their metabolic requirement by comparing it to a "real-life" spider of comparable activity levels, plus caloric requirement of silk production.
A Goliath Bid-Eater, the largest tarantula species by mass, weighs around 70-80 g on average (though it can grow to be 170 g). According to spider-care websites[2], a diet of 6-8 crickets per week is sufficient for "the larger tarantula species". That's only about 10 Calories a week!
By this calculation:
70 g spider => 10 Cal / week
∴ 70 kg spider => 10 * (1000^¾) Cal / week ~ 1780 Cal / week.
Let's take that number as "Caloric requirement at rest", since a captive Tarantula would have limited activity as compared to a 'wild spider' who would need to actively hunt for prey.
For spiders, a significant source of metabolic cost is also silk-creation. [1]. So the more web/silk a giant spider produces, the more calories it will need. Spider silk is light - enough spider silk to go around the world once would only weigh 500 g, but let's say these giant spiders spin proportionally thicker silk, so that the same length of silk weighs a 100 times more.
Caloric value of silk ~ 4500 Cal/g
So a 70 kg spider producing just 1 g of silk a day would need an additional caloric intake of 4500 Cal per day for silk production.
Since real spiders eat enough to tide them over for days at a stretch:
Assuming 70 Kg Spiders that produce 1 g Silk per day
And 100 Kg Deer that are 80% Consumable at 1.5 Cal/g with total 120K Cal
=> One (1) Deer would feed Four (4) Spiders for Seven (7) Days
ETA: However, even with a thread 10 times thicker (10 microns => 100 microns) and 100 times heavier than normal spider silk, 1 g of spider silk would be more than 166 m (1.31 g/cc /(0.005 cm *0.005 cm *pi)), so it is highly unlikely that even a giant spider would be producing 1 g of silk a day. [Thanks to @Sempie for bringing my attention to this].
So more realistically, unless silk production is significant enough (in terms of width/density/length) to weigh a lot, it would have insignificant calorie cost.
Additionally, Spiders often recycle their web silk. If these giant spiders are not expanding territory, they could be eating old silk to 'touch up' their webs, which could eliminate external (non-silk) caloric requirements even more.
Hence:
Assuming 70 Kg Spiders with negligible Silk production
And 100 Kg Deer that are 80% Consumable at 1.5 Cal/g with total 120K Cal
=> One (1) Deer would feed Thirty-Three (33) Spiders for Fourteen (14) Days
The paper linked above also gives an equation for activity cost of web building.
Total Cost of Web Construction ~ (4.5 * Weight of Web in mg) + [Weight of Spider in grams * (2.79 * Weight of Web in mg)] calories
I'm assuming this activity cost gets taken care of by Kleiber's Law, but if it doesn't, then the dietary requirements would be:
=> (4.5 * 1,000) + [70,000 * (2.79 * 1,000)] = 4500 + [70,000 * 2790]
= a whopping and unsustainable 195,304,500 Calories! (An adult African elephant needs around 70,000 Cal per day for comparison)