Reddit reviews First Book of Practical Studies: Cornet and Trumpet

The most economical thing to do would be to pick up an Olds Ambassador from eBay, you can get them for around $100 in good condition. To get back into playing you'll need to do a lot of long tones and lip slurs, my favorite complete warm up method was compiled by Greg Wing, based upon his studies with Bill Adams, found here. You could pick up an Arbans book for some etude playing, I also recommend the Getchell books, the etudes aren't too hard but can get tricky enough to be rewarding. That should get you started if you're serious you might want to think about picking up some other method books, Clarke, Schossberg, Irons, and Saint-Jacome are some of my favorites.