My suggestion is scour ebay and b&s for some good deals . FWIW, I am a bit classical in my pattern preferences but here is what I find to be versatile and essential. A navy blazer is a must. I would also recommend a glen plaid in brown, a herringbone in grey or brown, a houndstooth is brown or white/black, and a solid tweed (charcoal grey, brown, light grey etc). In the summer I would recommend a lighter navy blazer, maybe in a more "blue" color, a tan or brown check, and a silk/linen/wool/whatever mix in a neutral tone (light brown, navy, tan, light grey) to go with jeans or colored chinos.
As far as brands, this is highly personal. Polo can be found cheap, depending on your desire for more WASP-Y preppy patterns. Corneliani, Caruso, and Borrelli can also be found on deep discount. as always, you are likelly to find less staple pieces and more interesting patterns but you can always find something that will work with some creativity. I would recommend checking out howard yount as well, SF affiliate. Good product, good value.
I would find your best fitting jacket or have someone (a good tailor perhaps) and find your measurements. If you are a slim 40l you likely have about a 40-41 chest so depending on how tight you want your jackets chest to be, anything between 42-a little under 44 inches in the chest should be ok. Shoulder measurements are highly dependent on the structure of the shoulder (soft, padded, roping, no padding etc.) but there usually is a range of comfort (for me it is between 18.25-19 inches depending on the structure).
Once you know your measurements and how you prefer things to fit, just scour ebay, b&s, ehaberdasher, etc. and try to find jackets you think would work for your personality and compliment your shirts, ties, and pants. Good luck
I actually came to ask the exact question, so I appreciate the very helpful advice (as well as Steve Smith's). I'm in the same boat as the OP of the question... I'm a 40" chest and 18" shoulders... can anyone recommend some slimmer cut sportcoats/blazer brands to search for on eBay? I've started with Borrelli, Corneliani, PRL, but would appreciate any other input. Y'all are the best.