First thing you should do before looking for a pattern, is measuring yourself. It will ensure that you'll pick the right size. Most of sewing magazines/envelope pattern manufacturers/pattern websites should have a guide/list/table of measurements you need to take (like bust, hips, waist). You need to measure yourself in underwear. While measuring your hips, hold a long metal or wooden ruler for get a measurement with ease. Note them down and compare to sizes in the table, pick one closest to your measurements. If you happen to have bigger hips/bust pick a size up/down. In case of dresses, with multi-size pattern sheets you can combine two sizes into one.
jump to section: magazines, internet, envelope patterns, drafting your own patterns
Magazines (were and still) are most common way to source sewing patterns in Europe. Most of countries usually had own ones, but all over europe Burda Moden/Burda Style was most popular one for years, especially in Eastern Europe. It used to come with two pattern sheets in A0 size, printed on both sides though currently they have only one pattern sheet. Burda, since 90s has multi-size pattern sheets with every pattern in different color which makes copying easier compared to vintage ones which have ridiculously busy, confusing and cursed pattern sheets and certain pieces were only available in one or two sizes at most. Unfortunately they don't have seam allowance, which you need to add on fabric (not recommended) or on paper (it will ensure nicely, well cut pieces and you don't need to mark seam lines if you make them as wide as your presser foor).
Vintage magazines are easily available around, both on the internet and if you mom or grandma used to sew, you might already have them at home if you're an European.
pros: easy to find, low price per one pattern (one issue comes with 15-30 patterns), even used ones will have intact sheets you can use over and over to copy different sizes
cons:you need to copy a pattern from a pattern sheet and add seam allowance on your own, vintage magazines have very sprase sewing instructions and only maybe one or two models will have step by step instructions
Currently the easiest way to source sewing patterns is the internet. You can find both paid and free patterns. Paid patterns often are similar in price of envelope patterns, but you have to print them out and tape sheets together (if you can't get it printed on a plotter). They will come in bigger size range and they usually have seam allowances already on the pattern. You can download them in single size or multi-size variants.
Good places to find them are etsy and internet fabric stores might have digital downloads, additionally specialized sites offering patterns.
pros: easy to source, they can be obtained for free, come with seam allowance already drawn on pattern, more garments available in broader size ranges
cons: gluing together sheets might be a bit of hassle, high cost per pattern if paid, mostly modern styles or late 00s styles available only
Most common type of sewing pattern in North America. Most commonly they come from major companies like Butterick, Vogue, McCalls and Simplicty. I know that Burda also published them, but I barely seen them around in Europe. You need to cut your size out from provided pattern sheet. They often have alteration lines already marked that you can cut and change things around and there are high chances they come with step by step instructions. Patterns come in different body type ranges (misses, women's (plus size), petite or tall). Many companies often had designer's patterns (most notably Vogue Patterns), fitting dresses/pants which are simple patterns designed to check what you'll need to alter in other patterns as companies usually base majority of their patterns on their basic block.
There are loads of vintage patterns available, but finding an uncut sheet might be hard. If you can find cut one in your size or one bigger than you usual one, it still will be usable, if it wasn't altered.
pros: need to be only cut out to start sewing, clear alteration lines, clear (often step by step) instructions, lots of designer patterns available, fitting patterns which will make future alterations easier
cons: expensive per one pattern (unless it's a full set or has multiple variants), harder to find uncut vintage patterns
It's harder than using ready-made patterns, it has a higher learning curve but it allows you to create patterns on your exact measurements with great fit. After drafting basic bodice/skirt/pant blocks you can alter them as you want, creating new patterns. It takes more time as you will need to sew test garments or drape on dress form to see what needs to be fixed. You also can drape directly on dressform and then make a flat pattern.
pros: able to make a pattern fitted perfectly for your figure, many possibilities with altering patterns and moving darts, changing forms around
cons: has a higher learning curve, you need to test your blocks/patterns for perfect fit, takes more time to make a pattern and prepare for sewing, patternmaking books are expensive (unless you'll get them the other way)