Hi!
With all due respect, please get her a companion now!
Her species needs should at all times come before any human needs; that is basic welfare and acknowledged in the Five Animal Freedoms which have also been signed by the USA.
Also be aware that guinea pigs are prey animals and the majority of them are not suitable for what you are planning to do.
I know that using guinea pigs as care animals has unfortunately become currently a fad that sadly very often doesn't take respect of the personality of the piggies used. There are a few piggies that love meeting people and being fussed (I have met a very few); but the vast majority actually don't and is feeling very stressed by what you are planning to put her through.
Being docile doesn't equate as happy, just having their survival instinct running on high. Are you aware of how prey animal instincts work and can you read the body language and behaviour appropriately so you can give back to your piggy to balance the burden you are putting on her?
Are you saving up for vet care as a stressed single piggy is much more likely to develop illnesses due to a lowered immune system? Not having the emotional support from their own kind and a group they belong to is an added burden.
Our singles guide has a chapter on what expectations a single guinea pig is transferring to you lacking any company of their own kind. Have you considered that perspective?
Single Guinea Pigs - Challenges and Responsibilities
Understanding Prey Animal Instincts, Guinea Pig Whispering And Cuddling Tips
Arrival in a home from the perspective of pet shop guinea pigs
As to pellets: good quality pellets are not cheap, but as you only need to feed a tablespoon per day it is worth opting for a good timothy based brand. Good quality organic forage alternatives from shops is not cheap either; if you want to forage yourself for fresh plants, then you will have to be careful to stay away from dog peeing places (which is toxic for them), busy roads etc.
Please be aware that over 80% of the diet should be good quality hay. Fruit should be only used very sparingly as a treat. The sugar in them can help to promote dysbiosis (the gut microbiome going wrong, which can lead to bloating or diarrhea). Fresh herbs are a better alternative to fruit.
We are a UK based forum and can provide tips for suitable UK wild plants but we are not up to the flora in the USA or Canada. Since you are using 'pet store' and not 'pet shop', I assume that you are not UK based? It would rather help us if you mentioned your country when you are asking for advice that is based on your local availabilities.
Here is information on UK forage:
Safe wild weeds/plants you can feed your piggies