When buying perennial seeds from a seed company do they already come stratified or must I do it? I'm just wondering since most seed packets do not mention cold stratifying on their instructions.

Many perennials don't require cold stratification. Here's a good place to learn about germination requirements:
https://tomclothier.hort.net/
Have fun!

I hope the 2 cents I'm about to offer aren't too fuzzy, because although what I'm about to suggest has worked well for me in the past, I don't understand why. (The reader can skip the rest of this post and just look for hardy and half-hardy annuals, biennials and perennials relative to your zone).

Personally, I have had better luck with wintersowing some of the same plants that Tom Clothier's website recommends germinating at warm temperatures - in particular with respect to fungal and insect issues. In the winter time, when you put seeds outdoors in a container modified for wintersowing, some fungal and insect pests do not 'come out to play' and the gardener can let nature take over. That said, there are still some other things to pay attention to - like overheating and drying out and wind - but for me I basically put the prepared containers outdoors in winter and forget about them until temperatures start rising closer to the spring equinox in zone 7. The further south a gardener is, the trickier wintersowing becomes, but the principles are the same and gardeners just have to adjust how they wintersow or stratify accordingly.

Some of the plants recommended for sowing at warm temps on Tom Clothier's list that wintersowed well for me are:
ageratum
borage
browallia
dianthus
Salvia coccinea 'Snow Nymph'
tomato

Also, plants that can be germinated at both warm and cold temps are some of the ones that self sow, like Begonia semperflorens (wax begonia). Over the years, I double trenched every inch of my garden with composted weeds etc., and where I planted wax begonias and pentas in summer, seedlings would appear in summer the next year. They did not develop and flower as well as store-bought plants, but this does illustrate my point.

So, when selecting which seeds to wintersow (or stratify) relative to your zone, in addition to hardy perennials and biennials, look for the annuals described as hardy (HA) or half hardy annuals (HHA). Tom Clothier does tag some annuals that way in his list, but I do incorporate other sources as well, like the plant details on selectseeds.com . With regard to tender perennials that are treated like annuals in some places, I wait a few weeks after the winter solstice until about 4 - 6 weeks before the spring equinox to sow those seeds.

It's mainly my own garbling of sowing seeds via conventional methods that brought me to wintersowing, so I'm no expert. There is more expert advice on germinating seeds on the nga Wintersowing Form, especially in the sticky at the beginning: https://garden.org/forums/view...

I'm not aware of any commercial sellers that offer pre-stratification as standard procedure. Timing might render it impractical.

Once you've determined whether or not a particular selection benefits from stratification assume that you are the instrument of that process. If for some reason the seller had done it for you, they would state that.

Most seeds that do not require a cold stratification germinate fine if they get it anyway. Exceptions would be tropical species (of course), also to a lesser extent seeds that want high temperature 80+°F to germinate.

While dormant or semi-dormant bulbs can be pre-stratified when bought, seeds cannot. Cold stratification for seeds requires moisture, and a seed that has been primed to burst into action dies if it is dried for transport (or sprouts in transport if it is kept moist, since temperature is not controllable).

That said, the only unbroken rule in nature is that there are no unbroken rules.

Precisely this:

"While dormant or semi-dormant bulbs can be pre-stratified when bought, seeds cannot. Cold stratification for seeds requires moisture, and a seed that has been primed to burst into action dies if it is dried for transport (or sprouts in transport if it is kept moist, since temperature is not controllable).

That said, the only unbroken rule in nature is that there are no unbroken rules."

Was it Machiavelli who said that a wolf is to a sheep's clothing, as a broken rule is to a seemingly unbroken rule?

Please forgive me if my warped sense of humor goes awry here, but I do appreciate how well said the comments on this thread are.