My favorites are Ali Edwards, Studio Calico, Kelly Purkey, Feed Your Craft and In A Creative Bubble. All of these, except the last one, have monthly kits but you can always pick them up in the shop as regular purchases as well. Quite a few of them have themed kits most of the time, but if you're not subscribing you can wait and see if the theme appeals to you before picking it up. AE and KP have very specific styles that carry through month to month so you should check out if you like those styles and then wait for a theme that appeals to you. AE also has a kit that is themed around the month/monthly holidays so that is a really great one to pick up if you want just general themes. Studio Calico has the most cards in its kits compared to any of the other brands, so one kit goes a long way. Feed Your Craft has a few different designers that rotate month to month. Sometimes these brands will have grab bags for sale with an assortment of random cards that are usually a good deal.
In A Creative Bubble only has digital cards for sale in her shop so you have to print them yourself, but they are really well designed and you can browse her shop and find kits for any theme you're looking for. If you don't mind printing your own and are looking for a good deal, you might also want to check out the Becky Higgins Project Life App. I don't do digital project life, but the app lets you export cards on 4x6 canvasses and you can print them from there (either two 3x4s or one 4x6 card). You can print them with squared corners and kits are usually about $2-$3 each for A TON of designs. Not as well designed as other brands, and an annoying process to export the ones you want and then print yourself, but it is a good way to get a lot of designs for minimal cost. Plus, you have them digitally so you can print as many you want of any design and you can print as you need them so they don't take up a ton of space.