Welcome to the 450 community. Ammunition is getting easier to find on line at a little over $1 per round - see for example, https://ammoseek.com/ammo/450-bushmaster.
I suggest buying a couple of boxes to generate brass for reloading, rather than buying new brass. It will also help to evaluate how the gun shoots, without putting any reloading variable in play. As you can find out on this forum, a good basic load is 38 gr of Hodgden Lil Gun and 250 or 225 gr .452 FTX bullets, or their equivalent. Midway will occasionally sell "blems" with these specs, and they are very good value. Primers are a personal preference. I use Winchester Small Rifle; performance may be marginally enhanced with other brands. Most people use Hornady dies and omit the expander die (not needed) and taper crimp only.
Here are some bullets - https://www.midwayusa.com/s?userSearchQuery=450+bushmaster+bullets&userItemsPerPage=48.
Here's a link to Midways blemished bullet page, but 450's are not currently listed - https://www.midwayusa.com/s?userSearchQuery=Blemished+bullets&userItemsPerPage=48.
Hope this helps. There is a lot of info on this forum. Don't be afraid to ask.